Baseline (Widescreen) Solution & Issues
Widescreen resolutions are listed in-game. Menu backgrounds stretch from 4:3; everything else scales with the correct aspect ratio. FOV does not change with the resolution so has to be set manually to make gameplay Hor+ instead of Vert-.
To set your FOV edit ClientPrefs.cs in the config folder where the game is installed, then change $pref::PlayerFov to the FOV you want.
Here are some example FOV settings (used to make the screenshots on this page):
4:3: FOV 90
16:10: FOV 100
16:9: FOV 106
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues