Change font size of specific page in nav menu

font size:
easiest way is to create a custom template (http://codex.wordpress.org/Pages#Creating_Your_Own_Page_Templates) for that page, and run you’r own code. in that code wrap a div around all the content with a custom class and then change accordingly in your style.css file.

menu:
use a custom walker class – IE: extends the walker class (a nice tutorial here: http://www.kriesi.at/archives/improve-your-wordpress-navigation-menu-output).