Below you can see the front view size comparison of Fujifilm X-T3 and Sony A6400. Sony A6400 is 13mm narrower and 26mm shorter than Fujifilm X-T3 but it is also 1mm thicker.
Fujifilm X-T3 vs Sony A6400 - Front View
Fujifilm X-T3 vs Sony A6400 - 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