diff --git a/rendercanvas/_size.py b/rendercanvas/_size.py
new file mode 100644
index 0000000..ffaa1fe
--- /dev/null
+++ b/rendercanvas/_size.py
@@ -0,0 +1,71 @@
+class SizeInfo(dict):
+    """A dict with size information, for internal use.
+
+    Handy to have a separate dict, so that it can be passed to objects that need it.
+    Also allows canvases to create size callbacks without holding a ref to the canvas.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self["physical_size"] = 1, 1
+        self["native_pixel_ratio"] = 1.0
+        self["canvas_pixel_ratio"] = 1.0
+        self["total_pixel_ratio"] = 1.0
+        self["logical_size"] = 1.0, 1.0
+        self["need_size_event"] = False
+        self["need_context_resize"] = False
+
+    def set_physical_size(self, width: int, height: int, pixel_ratio: float):
+        """Must be called by subclasses when their size changes.
+
+        The given pixel-ratio represents the 'native' pixel ratio. The canvas'
+        zoom factor is multiplied with it to obtain the final pixel-ratio for
+        this canvas.
+        """
+        self["physical_size"] = int(width), int(height)
+        self["native_pixel_ratio"] = float(pixel_ratio)
+        self._resolve_total_pixel_ratio_and_logical_size()
+
+    def _resolve_total_pixel_ratio_and_logical_size(self):
+        physical_size = self["physical_size"]
+        native_pixel_ratio = self["native_pixel_ratio"]
+        canvas_pixel_ratio = self["canvas_pixel_ratio"]
+
+        total_pixel_ratio = native_pixel_ratio * canvas_pixel_ratio
+        logical_size = (
+            physical_size[0] / total_pixel_ratio,
+            physical_size[1] / total_pixel_ratio,
+        )
+
+        self["total_pixel_ratio"] = total_pixel_ratio
+        self["logical_size"] = logical_size
+
+        self["need_size_event"] = True
+        self["need_context_resize"] = True
+
+    def set_logical_size(self, width: float, height: float):
+        """Called by the canvas when the logical size is set.
+
+        This calculates the expected physical size (with the current pixel ratio),
+        to get the corrected logical size. But this *only* sets the logical_size
+        and total_pixel_ratio. The backend will, likely before the next draw,
+        adjust the size and call set_physical_size(), which re-sets the logical size.
+        """
+        # Calculate adjusted logical size
+        ratio = self["native_pixel_ratio"] * self["canvas_pixel_ratio"]
+        pwidth = max(1, round(float(width) * ratio + 0.01))
+        pheight = max(1, round(float(height) * ratio + 0.01))
+        lwidth, lheight = pwidth / ratio, pheight / ratio
+
+        # Update logical size and total ratio. You could see it as a temporary zoom factor being applied.
+        # Note that The backend will soon call set_physical_size().
+        self["logical_size"] = lwidth, lheight
+        self["total_pixel_ratio"] = self["physical_size"][0] / lwidth
+
+        self["need_size_event"] = True
+        self["need_context_resize"] = True
+
+    def set_zoom(self, zoom: float):
+        """Set the zoom factor, i.e. the canvas pixel ratio."""
+        self["canvas_pixel_ratio"] = float(zoom)
+        self._resolve_total_pixel_ratio_and_logical_size()
diff --git a/rendercanvas/base.py b/rendercanvas/base.py
index 4637d46..7a6e4a6 100644
--- a/rendercanvas/base.py
+++ b/rendercanvas/base.py
@@ -15,6 +15,7 @@
     CursorShapeEnum,
 )
 from . import contexts
+from ._size import SizeInfo
 from ._events import EventEmitter
 from ._loop import BaseLoop
 from ._scheduler import Scheduler
@@ -153,15 +154,7 @@ def __init__(
             if (self._rc_canvas_group and self._rc_canvas_group.get_loop())
             else "no-loop",
         }
-        self.__size_info = {
-            "physical_size": (0, 0),
-            "native_pixel_ratio": 1.0,
-            "canvas_pixel_ratio": 1.0,
-            "total_pixel_ratio": 1.0,
-            "logical_size": (0.0, 0.0),
-        }
-        self.__need_size_event = False
-        self.__need_context_resize = True  # True bc context may be created later
+        self._size_info = SizeInfo()
 
         # Events and scheduler
         self._events = EventEmitter()
@@ -224,7 +217,7 @@ def __del__(self):
 
     def get_physical_size(self) -> tuple[int, int]:
         """Get the physical size of the canvas in integer pixels."""
-        return self.__size_info["physical_size"]
+        return self._size_info["physical_size"]
 
     def get_bitmap_context(self) -> contexts.BitmapContext:
         """Get the ``BitmapContext`` to render to this canvas."""
@@ -323,41 +316,6 @@ def get_context(
 
     # %% Events
 
-    def _set_size_info(
-        self, physical_width: int, physical_height: int, pixel_ratio: float
-    ):
-        """Must be called by subclasses when their size changes.
-
-        Backends must *not* submit a "resize" event; the base class takes care of that, because
-        it requires some more attention than the other events.
-
-        The subclass must call this when the actual viewport has changed. So not in ``_rc_set_logical_size()``,
-        but e.g. when the underlying GUI layer fires a resize event, and maybe on init.
-
-        The given pixel-ratio represents the 'native' pixel ratio. The canvas'
-        zoom factor is multiplied with it to obtain the final pixel-ratio for
-        this canvas.
-        """
-        self.__size_info["physical_size"] = int(physical_width), int(physical_height)
-        self.__size_info["native_pixel_ratio"] = float(pixel_ratio)
-        self.__resolve_total_pixel_ratio_and_logical_size()
-
-    def __resolve_total_pixel_ratio_and_logical_size(self):
-        physical_size = self.__size_info["physical_size"]
-        native_pixel_ratio = self.__size_info["native_pixel_ratio"]
-        canvas_pixel_ratio = self.__size_info["canvas_pixel_ratio"]
-
-        total_pixel_ratio = native_pixel_ratio * canvas_pixel_ratio
-        logical_size = (
-            physical_size[0] / total_pixel_ratio,
-            physical_size[1] / total_pixel_ratio,
-        )
-
-        self.__size_info["total_pixel_ratio"] = total_pixel_ratio
-        self.__size_info["logical_size"] = logical_size
-        self.__need_size_event = True
-        self.__need_context_resize = True
-
     def add_event_handler(
         self, *args: EventTypeEnum | EventHandlerFunction, order: float = 0
     ) -> Callable:
@@ -380,23 +338,23 @@ def submit_event(self, event: dict) -> None:
 
     def __maybe_emit_resize_event(self):
         # Keep context up-to-date
-        if self.__need_context_resize and self._canvas_context is not None:
-            self.__need_context_resize = False
-            self._canvas_context._rc_set_size_dict(self.__size_info)
+        if self._size_info["need_context_resize"] and self._canvas_context is not None:
+            self._size_info["need_context_resize"] = False
+            self._canvas_context._rc_set_size_dict(self._size_info)
 
         # Keep event listeners up-to-date
-        if self.__need_size_event:
-            self.__need_size_event = False
-            lsize = self.__size_info["logical_size"]
+        if self._size_info["need_size_event"]:
+            self._size_info["need_size_event"] = False
+            lsize = self._size_info["logical_size"]
             self._events.emit(
                 {
                     "event_type": "resize",
                     "width": lsize[0],
                     "height": lsize[1],
-                    "pixel_ratio": self.__size_info["total_pixel_ratio"],
+                    "pixel_ratio": self._size_info["total_pixel_ratio"],
                     # Would be nice to have more details. But as it is now, PyGfx errors if we add fields it does not know, so let's do later.
-                    # "logical_size": self.__size_info["logical_size"],
-                    # "physical_size": self.__size_info["physical_size"],
+                    # "logical_size": self._size_info["logical_size"],
+                    # "physical_size": self._size_info["physical_size"],
                 }
             )
 
@@ -575,7 +533,7 @@ def get_logical_size(self) -> tuple[float, float]:
         The logical size can be smaller than the physical size, e.g. on HiDPI
         monitors or when the user's system has the display-scale set to e.g. 125%.
         """
-        return self.__size_info["logical_size"]
+        return self._size_info["logical_size"]
 
     def get_pixel_ratio(self) -> float:
         """Get the float ratio between logical and physical pixels.
@@ -585,7 +543,7 @@ def get_pixel_ratio(self) -> float:
         display-scale is set to e.g. 125%. An HiDPI screen can be assumed if the
         pixel ratio >= 2.0.
         """
-        return self.__size_info["total_pixel_ratio"]
+        return self._size_info["total_pixel_ratio"]
 
     def close(self) -> None:
         """Close the canvas."""
@@ -628,6 +586,9 @@ def set_logical_size(self, width: float, height: float) -> None:
         width, height = float(width), float(height)
         if width < 0 or height < 0:
             raise ValueError("Canvas width and height must not be negative")
+        # Already adjust our logical size, so e.g. layout engines can get to work
+        self._size_info.set_logical_size(width, height)
+        # Tell the backend to adjust the size. It will likely set the new physical size before the next draw.
         self._rc_set_logical_size(width, height)
 
     def set_title(self, title: str) -> None:
diff --git a/rendercanvas/glfw.py b/rendercanvas/glfw.py
index 38f36e4..03e6244 100644
--- a/rendercanvas/glfw.py
+++ b/rendercanvas/glfw.py
@@ -272,7 +272,7 @@ def _determine_size(self):
         pwidth, pheight = get_physical_size(self._window)
 
         self._pixel_ratio = pixel_ratio  # store
-        self._set_size_info(pwidth, pheight, pixel_ratio)
+        self._size_info.set_physical_size(pwidth, pheight, pixel_ratio)
 
     def _on_want_close(self, *args):
         # Called when the user attempts to close the window, for example by clicking the close widget in the title bar.
diff --git a/rendercanvas/jupyter.py b/rendercanvas/jupyter.py
index 96cfc2a..44eda92 100644
--- a/rendercanvas/jupyter.py
+++ b/rendercanvas/jupyter.py
@@ -107,7 +107,7 @@ def handle_event(self, event):
             pixel_ratio = event["pixel_ratio"]
             pwidth = int(logical_size[0] * pixel_ratio)
             pheight = int(logical_size[1] * pixel_ratio)
-            self._set_size_info(pwidth, pheight, pixel_ratio)
+            self._size_info.set_physical_size(pwidth, pheight, pixel_ratio)
             self.request_draw()
             return
 
diff --git a/rendercanvas/offscreen.py b/rendercanvas/offscreen.py
index ca861a7..29de76f 100644
--- a/rendercanvas/offscreen.py
+++ b/rendercanvas/offscreen.py
@@ -73,9 +73,9 @@ def _rc_present_bitmap(self, *, data, format, **kwargs):
     def _rc_set_logical_size(self, width, height):
         logical_size = float(width), float(height)
         pixel_ratio = self._pixel_ratio
-        pwidth = int(logical_size[0] * pixel_ratio)
-        pheight = int(logical_size[1] * pixel_ratio)
-        self._set_size_info(pwidth, pheight, pixel_ratio)
+        pwidth = max(1, round(logical_size[0] * pixel_ratio + 0.01))
+        pheight = max(1, round(logical_size[1] * pixel_ratio + 0.01))
+        self._size_info.set_physical_size(pwidth, pheight, pixel_ratio)
 
     def _rc_close(self):
         self._closed = True
@@ -100,7 +100,7 @@ def set_physical_size(self, width: int, height: int):
         """
         pwidth = int(width)
         pheight = int(height)
-        self._set_size_info(pwidth, pheight, self._pixel_ratio)
+        self._size_info.set_physical_size(pwidth, pheight, self._pixel_ratio)
 
     def set_pixel_ratio(self, pixel_ratio: float):
         """Set the pixel ratio, changing the logical size of the canvas.
@@ -110,7 +110,7 @@ def set_pixel_ratio(self, pixel_ratio: float):
         """
         self._pixel_ratio = float(pixel_ratio)
         pwidth, pheight = self.get_physical_size()
-        self._set_size_info(pwidth, pheight, self._pixel_ratio)
+        self._size_info.set_physical_size(pwidth, pheight, self._pixel_ratio)
 
     def draw(self):
         """Perform a draw and get the resulting image.
diff --git a/rendercanvas/pyodide.py b/rendercanvas/pyodide.py
index 4817b7a..7c2723b 100644
--- a/rendercanvas/pyodide.py
+++ b/rendercanvas/pyodide.py
@@ -203,7 +203,7 @@ def _resize_callback(entries, _=None):
 
             # Notify the base class, so it knows our new size
             pwidth, pheight = psize
-            self._set_size_info(pwidth, pheight, window.devicePixelRatio)
+            self._size_info.set_physical_size(pwidth, pheight, window.devicePixelRatio)
 
         self._resize_callback_proxy = create_proxy(_resize_callback)
         self._resize_observer = ResizeObserver.new(self._resize_callback_proxy)
diff --git a/rendercanvas/qt.py b/rendercanvas/qt.py
index 7f84679..6450131 100644
--- a/rendercanvas/qt.py
+++ b/rendercanvas/qt.py
@@ -584,7 +584,7 @@ def resizeEvent(self, event):  # noqa: N802
         # we can't. Not an issue on qt5, because ratio is always integer then.
         pwidth = round(lsize[0] * ratio + 0.01)
         pheight = round(lsize[1] * ratio + 0.01)
-        self._set_size_info(pwidth, pheight, ratio)
+        self._size_info.set_physical_size(pwidth, pheight, ratio)
         # self.update() / self.request_draw() is implicit
 
     def closeEvent(self, event):  # noqa: N802
diff --git a/rendercanvas/stub.py b/rendercanvas/stub.py
index f2a1e1b..2986fb8 100644
--- a/rendercanvas/stub.py
+++ b/rendercanvas/stub.py
@@ -59,27 +59,26 @@ class StubRenderCanvas(BaseRenderCanvas):
     """
     The ``RenderCanvas`` represents the canvas to render to.
 
-    Backends must subclass ``BaseRenderCanvas`` and implement a set of methods prefixed with ``_rc_``.
-    This class also shows a few other private methods of the base canvas class, that a backend must be aware of.
-    """
-
-    # Note that the methods below don't have docstrings, but Sphinx recovers the docstrings from the base class.
+    Backends must subclass ``BaseRenderCanvas`` and implement a set of methods
+    prefixed with ``_rc_``.
 
-    # Just listed here so they end up in the docs
+    Backends must call ``self._final_canvas_init()`` at the end of its
+    ``__init__()``. This will set the canvas' logical size and title.
 
-    def _final_canvas_init(self):
-        return super()._final_canvas_init()
+    Backends must call ``self._draw_frame_and_present()`` to make the actual
+    draw. This should typically be done inside the backend's native draw event.
 
-    def _set_size_info(self, physical_width, physical_height, pixel_ratio):
-        return super()._set_size_info(physical_width, physical_height, pixel_ratio)
+    Backends must call ``self._size_info.set_physical_size(width, height, native_pixel_ratio)``,
+    whenever the size or pixel ratio changes. It must be called when the actual
+    viewport has changed, so typically not in ``_rc_set_logical_size()``, but
+    e.g. when the underlying GUI layer fires a resize event.
 
-    def _process_events(self):
-        return super()._process_events()
-
-    def _draw_frame_and_present(self):
-        return super()._draw_frame_and_present()
+    Backends must also call ``self.submit_event()``, if applicable, to produce
+    events for mouse and keyboard. Backends must *not* submit a "resize" event;
+    the base class takes care of that. See the event spec for details.
+    """
 
-    # Must be implemented by subclasses.
+    # Note that the methods below don't have docstrings, but Sphinx recovers the docstrings from the base class.
 
     _rc_canvas_group = StubCanvasGroup(loop)
 
diff --git a/rendercanvas/wx.py b/rendercanvas/wx.py
index b698407..369d1ed 100644
--- a/rendercanvas/wx.py
+++ b/rendercanvas/wx.py
@@ -386,7 +386,7 @@ def _on_resize(self, event: wx.SizeEvent):
         # We use the same logic as for Qt to derive the physical size.
         pwidth = round(lsize[0] * ratio + 0.01)
         pheight = round(lsize[1] * ratio + 0.01)
-        self._set_size_info(pwidth, pheight, ratio)
+        self._size_info.set_physical_size(pwidth, pheight, ratio)
 
     def _on_key_down(self, event: wx.KeyEvent):
         char_str = self._get_char_from_event(event)
diff --git a/tests/test_base.py b/tests/test_base.py
index cedbbd2..74da38a 100644
--- a/tests/test_base.py
+++ b/tests/test_base.py
@@ -76,7 +76,7 @@ class MyOffscreenCanvas(rendercanvas.BaseRenderCanvas):
     def __init__(self):
         super().__init__()
         self.frame_count = 0
-        self._set_size_info(100, 100, 1)
+        self._size_info.set_physical_size(100, 100, 1)
 
     def _rc_get_present_methods(self):
         return {
@@ -153,7 +153,7 @@ def draw_frame():
     assert np.all(canvas.array[:, :, 1] == 255)
 
     # Change resolution
-    canvas._set_size_info(120, 100, 1)
+    canvas._size_info.set_physical_size(120, 100, 1)
 
     # Draw 3
     canvas.force_draw()
@@ -162,7 +162,7 @@ def draw_frame():
     assert np.all(canvas.array[:, :, 1] == 255)
 
     # Change resolution
-    canvas._set_size_info(120, 140, 1)
+    canvas._size_info.set_physical_size(120, 140, 1)
 
     # Draw 4
     canvas.force_draw()
diff --git a/tests/test_offscreen.py b/tests/test_offscreen.py
index ffcca1c..cf29daa 100644
--- a/tests/test_offscreen.py
+++ b/tests/test_offscreen.py
@@ -107,5 +107,33 @@ def test_offscreen_canvas_del():
     assert ref() is None
 
 
+def test_offscreen_extra_size_methods():
+    from rendercanvas.offscreen import RenderCanvas
+
+    c = RenderCanvas()
+
+    assert c.get_physical_size() == (640, 480)
+    assert c.get_logical_size() == (640.0, 480.0)
+    assert c.get_pixel_ratio() == 1.0
+
+    c.set_physical_size(100, 100)
+
+    assert c.get_physical_size() == (100, 100)
+    assert c.get_logical_size() == (100.0, 100.0)
+    assert c.get_pixel_ratio() == 1.0
+
+    c.set_pixel_ratio(2)
+
+    assert c.get_physical_size() == (100, 100)
+    assert c.get_logical_size() == (50.0, 50.0)
+    assert c.get_pixel_ratio() == 2.0
+
+    c.set_logical_size(100, 100)
+
+    assert c.get_physical_size() == (200, 200)
+    assert c.get_logical_size() == (100.0, 100.0)
+    assert c.get_pixel_ratio() == 2.0
+
+
 if __name__ == "__main__":
     run_tests(globals())
diff --git a/tests/test_size.py b/tests/test_size.py
new file mode 100644
index 0000000..b27c129
--- /dev/null
+++ b/tests/test_size.py
@@ -0,0 +1,160 @@
+"""
+Test size mechanics.
+"""
+
+from testutils import run_tests
+from rendercanvas._size import SizeInfo
+from rendercanvas.base import BaseRenderCanvas
+
+
+def test_size_info_basic():
+    si = SizeInfo()
+    assert si["physical_size"] == (1, 1)
+    assert si["logical_size"] == (1.0, 1.0)
+    assert si["total_pixel_ratio"] == 1.0
+    assert si["need_size_event"] is False
+
+    # Backends setting physical size
+    si.set_physical_size(10, 10, 1)
+
+    assert si["physical_size"] == (10, 10)
+    assert si["logical_size"] == (10.0, 10.0)
+    assert si["total_pixel_ratio"] == 1.0
+    assert si["need_size_event"] is True
+
+    # Different pixel ratio
+    si.set_physical_size(10, 10, 2.5)
+
+    assert si["physical_size"] == (10, 10)
+    assert si["logical_size"] == (4.0, 4.0)
+    assert si["total_pixel_ratio"] == 2.5
+    assert si["need_size_event"] is True
+
+
+def test_size_info_logical():
+    si = SizeInfo()
+    assert si["physical_size"] == (1, 1)
+    assert si["logical_size"] == (1.0, 1.0)
+    assert si["total_pixel_ratio"] == 1.0
+    assert si["need_size_event"] is False
+
+    # Base class setting logical size
+    si.set_logical_size(100, 100)
+
+    assert si["physical_size"] == (1, 1)  # don't touch!
+    assert si["logical_size"] == (100.0, 100.0)
+    assert si["total_pixel_ratio"] == 0.01
+    assert si["need_size_event"] is True
+
+    # Again
+    si.set_logical_size(640, 480)
+
+    assert si["physical_size"] == (1, 1)  # don't touch!
+    assert si["logical_size"] == (640.0, 480.0)
+    assert si["total_pixel_ratio"] == 1 / 640
+
+    # Now backend adjusts its actual size
+    si.set_physical_size(1280, 960, 2.0)
+
+    assert si["physical_size"] == (1280, 960)
+    assert si["logical_size"] == (640.0, 480.0)
+    assert si["total_pixel_ratio"] == 2.0
+
+    # Base class sets logical size again
+    si.set_logical_size(100.2, 100.2)
+
+    assert si["physical_size"] == (1280, 960)  # don't touch!
+    assert si["logical_size"] == (100.0, 100.0)  # took ratio into account
+    assert si["total_pixel_ratio"] == 1280 / 100
+
+    # And again
+    si.set_logical_size(101.8, 101.8)
+
+    assert si["physical_size"] == (1280, 960)  # don't touch!
+    assert si["logical_size"] == (102.0, 102.0)  # took ratio into account
+    assert si["total_pixel_ratio"] == 1280 / 102
+
+    # And backend adjusts its actual size
+    si.set_physical_size(204, 204, 2.0)
+
+    assert si["physical_size"] == (204, 204)
+    assert si["logical_size"] == (102, 102.0)
+    assert si["total_pixel_ratio"] == 2.0
+
+
+def test_size_info_zoom():
+    si = SizeInfo()
+    si.set_physical_size(1200, 1200, 2.0)
+
+    assert si["physical_size"] == (1200, 1200)
+    assert si["logical_size"] == (600.0, 600.0)
+    assert si["total_pixel_ratio"] == 2.0
+
+    # Adjust zoom
+    si.set_zoom(3)
+
+    assert si["physical_size"] == (1200, 1200)
+    assert si["logical_size"] == (200.0, 200.0)
+    assert si["native_pixel_ratio"] == 2.0
+    assert si["total_pixel_ratio"] == 6.0
+
+    # Backend updates its size, zoom is maintained
+    si.set_physical_size(1800, 1800, 2.0)
+
+    assert si["physical_size"] == (1800, 1800)
+    assert si["logical_size"] == (300.0, 300.0)
+    assert si["native_pixel_ratio"] == 2.0
+    assert si["total_pixel_ratio"] == 6.0
+
+    # Backend updates its physical size
+    si.set_physical_size(600, 600, 1.0)
+
+    assert si["physical_size"] == (600, 600)
+    assert si["logical_size"] == (200.0, 200.0)
+    assert si["native_pixel_ratio"] == 1.0
+    assert si["total_pixel_ratio"] == 3.0
+
+    # Adjust zoom last time
+    si.set_zoom(2)
+
+    assert si["physical_size"] == (600, 600)
+    assert si["logical_size"] == (300.0, 300.0)
+    assert si["native_pixel_ratio"] == 1.0
+    assert si["total_pixel_ratio"] == 2.0
+
+
+class MyRenderCanvas(BaseRenderCanvas):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self._requested_size = None
+        self._final_canvas_init()
+
+    def _rc_set_logical_size(self, w, h):
+        self._requested_size = w, h
+
+    def apply_size(self):
+        if self._requested_size:
+            self._size_info.set_physical_size(*self._requested_size, 1)
+            self._requested_size = None
+
+
+def test_canvas_sizing():
+    c = MyRenderCanvas(size=None)
+
+    assert c.get_logical_size() == (1.0, 1.0)
+
+    c = MyRenderCanvas()
+
+    assert c.get_logical_size() == (640.0, 480.0)
+    assert c.get_physical_size() == (1, 1)
+    assert c.get_pixel_ratio() == 1 / 640
+
+    c.apply_size()
+
+    assert c.get_logical_size() == (640.0, 480.0)
+    assert c.get_physical_size() == (640, 480)
+    assert c.get_pixel_ratio() == 1.0
+
+
+if __name__ == "__main__":
+    run_tests(globals())