Below you can see the front-view size comparison of the Panasonic S1R and Fujifilm GFX 50S. The Fujifilm GFX 50S is clearly the smaller of the two cameras. Its body is 1 mm narrower, 16 mm shorter and 6 mm thinner than the Panasonic S1R.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.