Yeah, it's an XFree86 config thing, regardless of the dektop environment. If you specified multiple resolutions during your X setup, Thymox's advice will allow you to toggle through those resoultions on the fly.
If your desktop really doesn't fit on the screen, but you can "scroll" to the off-screen parts by pushing your mouse against the screen boundaries, then you have the "virtual desktop" feature enabled. You can disable this feature by checking the Screens section of your XF86Config file for a lines which begins with "Virtual". Comment it out and save the file.
If you edit this file, make a backup copy of it first!! If you mess up the file, your X install stands a very good chance of getting hosed.
I think there is also a way to disable it from within KDE or Gnome. I can't remember where, and I'm away from my Linux boxen right now.