One site I have noticed is good at this is
Amazon. If you change the font size when looking at their web site (I am using IE6; if I hold down the "Ctrl" key I can use the mouse wheel to move up and down through the sizes) I think it looks pretty good on all the settings - they manage to maintain a nice layout.
If you do a "View Source" on the page, you can see the CSS commands - they mostly use "font-size: small".
I guess you don't have absolute control over font size if you can't specify pixels, but then you don't have control over users' screen sizes and resolutions, so maybe it's just as well.