In Video.con
game.setGameDisplayMode 1200 800 32 75
This setting controls the resolution, color depth and refresh rate of the in-game image. You can alter this using the in-game Display Mode setting, however if you want to choose a non-standard resolution and/or refresh rate (e.g. Widescreen), set it here. Make absolutely certain your display device supports the resolution and refresh rate you are selecting otherwise you may end up with an 'out of frequency' error or black screen, and even cause damage to your display.
Note, the resolution will change for in-game gameplay as specified above, however the main menu resolution will always remain the same and cannot be changed.