Below you can see the front-view size comparison of the Fujifilm X-T3 and Fujifilm GFX 50R. The Fujifilm X-T3 is clearly the smaller of the two cameras. Its body is 28mm narrower, 4 mm shorter and 7 mm thinner than the Fujifilm GFX 50R.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.