Okay, so I have a 4k monitor, and that's the resolution I'm playing on, with UI scaled to 2x, all perfect.
All good 'n stuff, but when I load a game it sets it back to 1980x1080, but no worries, I just go into options, and set it back, takes only a couple seconds.
What I've done though, is that one time I loaded a game without first setting back the resolution, and it further reduced it, to the point that it's massively zoomed into the center, with most of stuff missing, I can go into options, but neither the UI scaling or the resolution options are accessible, and I'm completely locked out of playing.
Resolved by myself,
in regedit
HKEY_CURRENT_USER\Software\Ludeon studios
I had 2 seperate folders "RimWorld" and "RimWorld by Ludeon Studios"
However I changed the first one, it never applied, until I realized the second one contains the same config files, and once I edited those, it worked.
Edit: Alternate method someone told me
"In the config folder there is a prefs.xml file.
Open that file with any text editor and you can change the UI scale setting back to 1"
I believe the "RimWorld" entry in regedit is the old path used before Alpha 16. You can delete it or ignore it - it isn't used for anything now. Just don't delete the "RimWorld by Ludeon Studios" registry entry!