You may need to manually edit a file. The file is /etc/X11/xorg.conf
. Open it up as root or you won't be able to edit it. Look at the "Section Screen". It should look like this:
Identifier "Default Screen"
Device "NVIDIA Corporation NV43 [GeForce 6600/GeForce 6600 Ultra]"
Monitor "CPD 4410"
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
The file is quite self-explanatory. Add your preferred resolution as the first option. (above, my first option is 1600x1200). Edit, save the file and restart the Xserver. That should do it
Tell me if any steps are unclear. I'm not sure how much you can with Linux, so this can either be too difficult or just enough
P.S: always remember to backup stuff, just in case something goes wrong