You can either use the xrandr command-line tool this shows you the names of different outputs available on your system (LVDS, VGA-0, etc.) and resolutions available on each.
sudo su
xrandr
Or You can direct xrandr to set a different resolution using the following command
sudo su
xrandr --output VGA --mode 800×600 --rate 75
Due to buggy hardware or drivers, your monitor's correct resolutions may not always be detected. If the mode already exists, but just isn't associated for the particular output using the following command
sudo su
xrandr --addmode VGA 800×600
There are several ways to make xrandr customizations permanent from session to session:
1- A user's ~/.xprofile file is executed on Xorg startup if it exists and is executable. You can copy and paste xrandr command line strings into this file so they're executed when you log in.
2- Both kdm/gdm/lightdm have startup scripts that are executed when X is initiated. In either case, you can paste in an xrandr command line string into one of these scripts.
3- xorg.conf, see man xorg.conf for full details on how to use an xorg.conf file.
Check man page from here:
http://www.x.org/archive/X11R6.8.0/doc/xorg.conf.5.html