Below you can see the front-view size comparison of the Nikon D5 and Fujifilm GFX 50S. The Fujifilm GFX 50S is clearly the smaller of the two cameras. Its body is 12 mm narrower, 65 mm shorter and 1 mm thinner than the Nikon D5.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.