Do you know css?
You can edit your style.css file. But first, you’ll need to know what is the class name or id for the dropdown menu.
If you use Firebung (Firefox plugin), you can see what is the name of the dropdown menu class or id.
After getting YOUR-NAME with Firebug, go to your css file, and find something like:
.YOUR-NAME ul or .YOUR-NAME or #YOUR-NAME ul or #YOUR-NAME and edit the width attribute.
I think that will solve the problem