In your case, you could have multiple stylesheets and swap which one is linked to your page with server side code.
OR
You could use ASP.NET 2.0 themes/skins.
If you look on my website, in the upper right corner there is a theme selector. I'm storing the theme using ASP.NET's profile provider. This works for anonymous users too. I change color schemes but you could do something similar with font size.
Matt Brown