Incidentally, this hex editing people are doing isn't exactly necessary. If you hook
GetSystemMetrics (...) and change a few values, you can make the game happy enough to create a window that wouldn't normally fit on screen.
Resolutions like 3440x1440 are greyed-out because the game requests the monitor's working area (sans taskbar), subtracts space for the border and title and then if it thinks the window will fit on screen, offers it in the list.
It uses the same set of resolutions for fullscreen, just adds that secondary test condition to widdle the list down. Spoofing certain metrics fixes both HUD scaling and the number of resolutions available in windowed mode.
I don't expect many to understand what I just wrote, but I'll put it this way ... I can do the same thing without editing any game memory and that should keep anti-cheat systems very happy