Below you can see the front view size comparison of Nikon D850 and Sony A99 II. Sony A99 II is clearly the smaller of the two cameras. Its body is 3mm narrower, 20mm shorter and 3mm thinner than Nikon D850.
Nikon D850 vs Sony A99 II - Front View
Nikon D850 vs Sony A99 II - 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