Maybe you could use this in command line
xrandr --newmode "1024x768" 64.11 1024 1080 1184 1344 768 769 772 795
-HSync +Vsync
and then this
xrandr --addmode VGA 1024x768

or if you want another screen resolution you could try generate another
modeline using modeline calculator in
http://www.arachnoid.com/modelines/index.html

after adding mode you should have change your monitor resolution using
menu->System->Preferences->Display and chose new resolution that you
just add using xrandr command.