how to change main menu font size

Hey guys – i’ve done a bit of searching on this forum and found this code to change the main menu font size – but it’s not working.

.main_menu ul:first-child > li > a{ font-size: 12px !important; }

can you help me – I need to change the font size as I’m adding another item and it will crowd the header.

thanks

http://www.acmena.com.au/