Site Title in Header, two different font sizes possible?


I am using Thanks Roy, and I want to have (in the header) a big title and a longer subtitle in a smaller font size underneath.
where can I do this?
In Settings I can only insert a title and have no option to choose size or Even to a line break.
In Appearance /configure Theme is also no possibility…
would this change be in CSS Editor, if so, what would be the necessary code?