With OS X, someone told me that modern Macs now display fonts at the same size as PCs. In the past, the font size was nearly unreadable on Macs when it was average size on a PC (without a special script changing 12pt to 16pt for Macs).

My question is this - Is it okay these days to leave <font size=2> and expect Mac users to be able to read it at the same size as Windows users? Or should I use a more complicated CSS scheme?

A CSS scheme would be much less complicated than declaring fonts using the <font> tag for every single different font you want to display on the screen. It's also much more user-friendly, especially for people with systems that require making adjustments to read a screen properly. Absolutely, positively avoid using the <font> tag at all costs. Go the CSS route.

Thanks for the help, the_pm... another question, those using CSS, is it necessary these days to specify a different px or pt for different browsers/platforms? Or are Macs and Linux now in synch with the Windows display standards? (96dpi vs. 72dpi, etc.)

It's difficult to specify a font size for different OSs, and really it shouldn't be necessary. Just keep your font sizes relative (use percentages) and don't go too small :)