I've been digging a bit on this, and it would seem that it is unfortunately not possible, because browsers are limited in their handling of font sizes in the rich text editors. In particular, Firefox generates the (deprecated) <font size="x"></font>
element when the font size is changed, and the value of x can be only in the 1-7 range.
If you have a look at the setFontSize method in RichTextAreaImplStandard (GWT source code), you'll see that it ends up calling the execCommand javascript function, which in the case of FontSize only accepts values in 1-7:
http://msdn.microsoft.com/en-us/library/ms536991%28VS.85%29.aspx