Depends also on the font. I like keeping Arial at 12px or larger. Verdana is wider and you could take it down to 11px or 10px... but I hate to use fixed px sizes really small because users can't resize them.
This, in my opinion, is completely the wrong way to look at this. It really is a usability issue, and that to me trumps everything.
If you're thinking that an 8px font is more usable than scrolling, you're not thinking very clearly. ;)