provided that you do things right, changing the UI back to default is a simple matter of changing a single line in a file (or deleting it alltogether).
That being said, there's always a risk of doing something bad (tm) to your UI.
Best bet is to backup the UI cache on your char, as well as your UI\Default folder. Should be safe enough.
Most of the mods in my experience, size pretty well, if not exactly. Statbars as an example seem to be of the 1024 length variety at the moment, although I (am possibly others) are working on learning how to size things automatically.
In all, read the FAQ's and the stickies on these forums, then pick something simple, and try to install it. Something like
Insom's scrollbarless chatwindows is very simple to install and test for instance.