Do you mean smallest screen size, or lowest quality.
I don't really think that low quality will hinder your ability to aim, but a small screensize will.
Assuming you do have a small screen size, and not low quality, since it is almost impossible to see the menu at such a size, go to your folder that has the autogen.cfg and delete it.
(Correct me if i'm wrong, but I think that should reset your screen size to the default 640x480)
To get there, go to
\Documents and Settings\YOU\Local Settings\Application Data\Tremulous\base\
where "YOU" is your windows username.
[Instructions stolen from TJW]