Below you can see the front view size comparison of Nikon D810 and Pentax 645Z. Nikon D810 is 10mm narrower and 41mm thinner than Pentax 645Z but it is also 6mm taller.
Nikon D810 vs Pentax 645Z - Front View
Nikon D810 vs Pentax 645Z - 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