Below you can see the front view size comparison of Sony A6000 and Panasonic GH3. Sony A6000 is clearly the smaller of the two cameras. Its body is 13mm narrower, 26mm shorter and 37mm thinner than Panasonic GH3.
Sony A6000 vs Panasonic GH3 - Front View
Sony A6000 vs Panasonic GH3 - 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