Sub Menu Width too narrow - how to change?

I am using the Network theme for this site http://graniteshoplistings.com and if you go to the site and place your mouse over the "Classifieds" top menu and you will see that the second sub menu "My Classifieds" is broken into two lines. This means the sub-menu is too narrow. I need to increase the width or set it so that no wrapping is occurred. Which file should I look at?