Below you can see the front view size comparison of Canon 5D MIII and Pentax K-1. Pentax K-1 is 15mm narrower and 6mm shorter than Canon 5D MIII but it is also 10mm thicker.
Canon 5D MIII vs Pentax K-1 - Front View
Canon 5D MIII vs Pentax K-1 - 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