It's in this file. You should be able to do a quick search for the resolution you're currently using, then change it.
One quick note:
As far as I have been able to tell (note that I'm no expert in Linux...), you cannot change the desktop resolution without restarting X. While you can use Ctrl+Alt++ and Ctrl+Alt+- to change the screen resolution (if more than one resolution is defined in the XF86Config-4 file), the desktop remains at the highest resolution defined (virtual desktop sorta thingy).
"Physics is like sex. Sure, it may give some practical results, but that's not why we do it." - Richard P. Feynman