Change Menu text size? Or collapse subpages?

Hi,

I love this theme, it’s exactly what I needed! I’ve been recently adding new pages and now my menu is a bit too big. The last few page links disappear off the bottom of the screen. Is there a way to make the text in the menu smaller? Or maybe to collapse/expand subpages?

Sorry if these are dumb questions, I really don’t know much about CSS!

Thanks for your help.