How do I change the screen resolution in X?

If you have more than one resolution defined for X, you can change your screen resolution by hitting the following key combinations:

• Ctl-Alt-plus increases the screen resolution (makes everything smaller);
• Ctl-Alt-minus decreases the screen resolution (makes everything bigger).

If this doesn't work, then you are going to have to define more video modes in /etc/X11/XF86Config. Read the man page for XF86Config to do it manually (which might well be the fastest way to do it), or failing that reconfigure your X completely from scratch by using either XF86Setup or xf86config. If you take this route, back up your existing X config file incase you completely break X.