First, I hope you've created your own template and aren't modifying the files in the /classic/ folders. If you modify the files in the /classic/ or /template_default/ folders, all of your changes will be overwritten (LOST) during an upgrade/update.
To answer your question, to change that line look in your stylesheet for:
Code:
#navMain ul, #navSupp ul, #navCatTabs ul {
margin: 0;
padding: 0.5em 0em;
list-style-type: none;
text-align: center;
line-height: 1.5em;
}
You can adjust the line-height accordingly. You may need to adjust the font size elsewhere, but probably only if you're planning on making that line very small.