you can force various video resolutions by editing ~/.doom3/base/AutoExec.cfg (create the file if it doesnt exist)
Add a line
where
x can be a numeric value between 0 and 8.
The following list gives the video modes for each value
0 = 320x240
1 = 400x300
2 = 512x384
3 = 640x480
4 = 800x600
5 = 1024x768
6 = 1158x864
7 = 1280x1024
8 = 1600x1200
HTH
Mad.