Hi Arnd!

just a little hint - there is a HTML Tag <small> that's even valid in strict.

Sure. And there's already a BBCode for it (and an other one for large text). Example:
NORMAL SMALL [color=#00f]LARGE[/color]

Syntax:
[size=small]foo[/size]
[size=large]bar[/size]

For a list of BBCodes see this post.