Below you can see the front view size comparison of Sony A7R II and Nikon Z6. Sony A7R II is clearly the smaller of the two cameras. Its body is 7mm narrower, 5mm shorter and 8mm thinner than Nikon Z6.
Sony A7R II vs Nikon Z6 - Front View
Sony A7R II vs Nikon Z6 - 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