diff --git a/fsspec/callbacks.py b/fsspec/callbacks.py
index 7ca99ca6a..9354ee1b2 100644
--- a/fsspec/callbacks.py
+++ b/fsspec/callbacks.py
@@ -91,8 +91,10 @@ def set_size(self, size):
 
         Parameters
         ----------
-        size: int
+        size: int or callable
         """
+        if callable(size):
+            size = size()
         self.size = size
         self.call()