Same problem here. Have just upgraded to GP 2013, and one user has master file and transaction screens that are about 1/2 the size they used to be, and are smaller than the same screen on other users' computers. Other users have the same normal size screens.
We've seen the suggestion about modifying the Windows screen resolution, but that would affect everything, not just Dynamics. The user just doesn't see the need to do that.
Also, regarding changing the Dex.ini file, we've looked in there, but didn't see anything pertinent. Maybe the Tolerance setting?