Below you can see the front view size comparison of Sony A7 III and Nikon Z6. Sony A7 III is 7mm narrower and 5mm shorter than Nikon Z6 but it is also 6mm thicker.
Sony A7 III vs Nikon Z6 - Front View
Sony A7 III 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