Below you can see the front view size comparison of Nikon D810 and Pentax K-1. Pentax K-1 is 9mm narrower and 13mm shorter than Nikon D810 but it is also 4mm thicker.
Nikon D810 vs Pentax K-1 - Front View
Nikon D810 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