there are many ways of specifying a font size using css:

named sizes eg. medium, small, large always display smaller on macs.
as b. donnely says in a previous message the 2 platforms translate point 
sizes differently causing macs to display point sizes smaller.
if you specify pixel sizes this should solve your problem eg.

<style type="text/css">
<!--
.pixels10 { font-family: Arial, Helvetica, sans-serif; font-size: 10px}
-->
</style>]

should be readable on all platforms/browsers

joe