Right click on the short cut you use to start tremulous, click properties, it should have a target field saying something like "C:\Program Files\Tremulous\Tremulous.exe", add "+set r_mode 4" to the end.
It should then run at a lower res.
Probably best to change the link back after you've run it, or you'll be unable to change res in-game.
Once the game is running type "/fs_homepath" in the console to see where it has your autogen etc saved, it's always useful to know.