The Asp.net menu control adds font-size:1em to the menu items.

This prevents setting the font-size in a CSS style sheet. That is a nuisance.

The quickest way to override this problem is by using the !imporant modifier in your CSS style sheet like this.

a.my_menu_style 
{ font-size:2em !important; }

To gain more control over the HTML that Asp.net controls output, see this page Asp.Net 2.0 CSS Friendly Control Adapters.