Have a look in your /etc/xorg.conf file. It should have a section similar to this one:
Identifier "Default Screen"
Device "S3 Inc. ProSavage KN133 [Twister K]"
Monitor "Generic Monitor"
# Skipping some text to improve readability
After "Modes" you can add other modes, e.g.:
Modes "1024x768" "800x600"
Then you switch between modes with <ctrl+alt+plus on num pad> ...
A detailed description is below.
Modes N'34'mode-nameN'34' ...
This optional entry specifies the list of video modes to use. Each mode-name
specified must be in double quotes. They must correspond to those specified or referenced in the appropriate Monitor
section (including implicitly referenced built-in VESA standard modes). The server will delete modes from this list which don't satisfy various requirements. The first valid mode in this list will be the default display mode for startup. The list of valid modes is converted internally into a circular list. It is possible to switch to the next mode with Ctrl+Alt+Keypad-Plus
and to the previous mode with Ctrl+Alt+Keypad-Minus
. When this entry is omitted, the valid modes referenced by the appropriate Monitor
section will be used. If the Monitor
section contains no modes, then the selection will be taken from the built-in VESA standard modes.