This game's fullscreen mode is a type of borderless window, so the in-game resolution option determines the internal resolution. Resolutions with mismatched aspect ratios will be letterboxed/pillarboxed.
The first time the game is run it stores the desktop resolution. The in-game resolution list shows this stored resolution plus a hard-coded list of resolutions. Listed resolutions are 640x480, 720x480, 720x576, 800x600, 1024x768, 1152x864, 1176x664, 1280x720, 1280x768, 1280x800, 1280x960, 1280x1024, 1360x768, 1366x768, 1600x900, 1600x1024, 1680x1050, 1768x992, 1440x900, 1920x1080, 2048x1280, 2560x1600, 3840x2160, 4096x2160. The recorded desktop resolution is shown last in the list (resolution slider positioned to the far right).
Unlisted resolutions can be set with command line arguments (e.g. -screen-width 2560 -screen-height 1080) or by editing the registry: HKEY_CURRENT_USER\SOFTWARE\Overflow Games
