the TFix full and TFix lite executables are identical, so their capability to display the game in a certain resolution is also the same. TFix full tries to check for your desktop resolution during install and sets the game to that if it succeeds, that's the only difference. so with TFix lite, you need to open the video settings from the main menu and set whatever resolution you like there - most, if not all standard resolutions should be exposed.

are you using something unusual perhaps? if so, both cam.cfg and cam_ext.cfg will have to be edited to add the extra resolution.

also, Thief mod loading got a whole lot easier recently, check here for all available Thief mods.