There are a few ways to change the "text scaling factor" http://askubuntu.com/questions/60044/how-do-i-change-the-fon... but that doesn't change the real DPI that X uses. If your hardware supports XRANDR, it should automatically configure your device. If it gets a wrong value or you just want to override it: http://wiki.debian.org/XStrikeForce/HowToRandR12#III.3._Chan...