Below you can see the front view size comparison of Sony A9 and Nikon D810. Sony A9 is clearly the smaller of the two cameras. Its body is 19mm narrower, 27mm shorter and 19mm thinner than Nikon D810.
Sony A9 vs Nikon D810 - Front View
Sony A9 vs Nikon D810 - 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