[Phanx]

There is a maximum absolute size at which WoW will render a font string. For most fonts, this maximum absolute size (in pixels) is reached at a font size (I guess it's points?) of 30-32. If your font is topping out at 22, it's probably oddly designed, with lots of whitespace around each character. Without seeing the actual font file you are using though, I can't say for sure whether that is the problem, but a value of 48 or 9000 will have the same visual effect as a value of 32 even for a "normal" font.

@Wimpface:
Calling SetFont overrides any base font object you set, either by passing a font object to CreateFontstring, or by calling SetFontObject explicitly. Once SetFont is called, your fontstring is no longer considered to "have" a font object (though GetFontObject might still return something) and changes to the font object will no longer propigate to your fontstring, though any properties (like shadows) you don't override will be retained. So, passing a font object to CreateFontstring is not the cause of OP's problem.