Usually XGUI displays the whole label, then just makes the slider smaller. Recently, the gmod slider default layout code was changed to set the label to a fixed size, which I have since fixed in a commit last week. Try updating to the latest ULX version via the git/svn link, and see if it fixes your problem.
Also, I can't remember if I put a tooltip/hint in there or not, but if you hover over the slider, it might show you the full text. I think.