General Support: Re: Title/Organisation Name Colour
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)
I was adding it into custom CSS in the theme settings, the other changes did change on re-load (colour/font).
Will give it another shot.