Help edit size font

Hello, i am from Argentina, i have a problem… I can´t found how edit the size font of my navigation menu…
The code of navigation menu is:

<ul id="nav">
        <?php wp_list_pages('sort_column=menu_order&title_li='); ?>
</ul>

That shows me all the pages of wordpress, the problem is that do not know where to modify the font size!
I check all the code of style.css and find nothing referring to”id=nav”

As would have to appear supposedly in the style.css?
Maybe not this part of code isn’t create yet… As you could edit the size? : (

Thanks and sorry for my bad english!