Change post title size

I have a sidebar displaying a custom menu and would like to display menuitems in different sizes. The tricky part is that i want to be able to assign 1 of 3 sizes from the post edit screen when i create the posts. Ive created a custom field for that purpose that outputs 3 different names i can use for css classes. But how do i add a class from a custom field to nav menu item?

All help is appreciated! 🙂