Below you can see the front view size comparison of Sony A7 III and Nikon D850. Sony A7 III is clearly the smaller of the two cameras. Its body is 19mm narrower, 28mm shorter and 5mm thinner than Nikon D850.
Sony A7 III vs Nikon D850 - Front View
Sony A7 III vs Nikon D850 - 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