Apply suggestions from code review

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

Author: core-man

pygmt/tests/test_plot3d.py

@@ -335,8 +335,8 @@ def test_plot3d_varying_intensity():
         y=y,
         z=z,
         region=[-1.1, 1.1, -0.5, 0.5, -0.5, 0.5],
-        projection="X15c",
-        zscale=0.1,
+        projection="X15c/5c",
+        zsize="5c",
         perspective=[135, 30],
         frame=["Sltr", "xaf+lIntensity"],
         style="c0.5c",