Below you can see the front view size comparison of Sony A6400 and Canon 90D. Sony A6400 is clearly the smaller of the two cameras. Its body is 21mm narrower, 38mm shorter and 17mm thinner than Canon 90D.
Sony A6400 vs Canon 90D - Front View
Sony A6400 vs Canon 90D - Top View
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.
Camera Size Comparison
Select Two Cameras for Size Comparison