I would strongly discourage you from doing this. People who have the font size set at a different viewing size to 'normal' often do so because of genuine reasons - often because they have a visual impairment. My view (and that of many others including the RNIB) is that the Internet should be accesible to ALL and web designers should do everything they can to allow this to happen. Allowing people to view fonts in a different size is one way this can be done.
I know your site may look odd and the 'nice' design may be lost but that should be the visitors choice. Most will know if they have a massive font in use the site will look different to how it was designed.
I realise that bit doesn't answer your question ;)
I think if you use fixed font sizes (pt and not %) then they will remain fixed regardless of a users setup. You don't have to use CSS for this.