Below you can see the front view size comparison of Fujifilm X-T3 and Pentax K-3. Fujifilm X-T3 is 7mm shorter and 18mm thinner than Pentax K-3 but it is also 2mm wider.
Fujifilm X-T3 vs Pentax K-3 - Front View
Fujifilm X-T3 vs Pentax K-3 - 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