font-size: 50px; (or whatever size you like) should work fine - I just tried it in the web developer tools in Firefox and the size changed as expected.

If you're adding it to a custom css file you may find it doesn't update straight away as the file can be cached. Maybe try it and be patient to see if it changes after a while.

(though you could instead put the custom CSS into the theme settings where any changes will appear instantly when you reload any page)

Thanks

Joe

Hmm, just tried again and it worked straight off, not sure what was up this afternoon, but was trying for ages, anyway works now so all good.  Cheers