How large is your screen? I’m asking because your screenshot has a 2942x1638 resolution, but the info shown in the RawTherapee options files says: 1464x748. OK, that needs a little extra room for the workspace header and the window manager, but it isn’t even close to the aforementioned screenshot size. It is, in fact, a factor 4 smaller and exactly what you are seeing. Maybe you can force it:
Assuming for the moment that 2942x1638 is your screen size: (Stop RT first) Try editing the options file and change:
WindowWidth=1464
WindowHeight=748MeowWidth=1200
MeowHeight=680
to:
WindowWidth=2560
WindowHeight=1440
MeowWidth=2560
MeowHeight=1440
The actual numbers are an approximation and well within the screenshot size, I don’t have a clue how much is taken by MacOS’ WM (in Linux I can use the full screen resolution). If this works you might want to play with these numbers.
Anyway; Not sure what is going on here and I’m still not certain that this is an actual RawTherapee related issue. The screen size (workspace size on a Mac?) is provided by the OS and picked up by RT when first starting the program. Something seems to have gone wrong there. I do not have the MacOS expertise to help you with the MacOS bit, though.