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I'd guess medium with a pretty signficant number selecting the next higher font size. Micro fonts are one of the most often complained about problems with the v5+ browser set.
Besides, if you use css to set the font size for ie, there is nothing they can do to get around that.
If you use px units or pt which are fixed font units IE users can't resize.
If you use em, or any other relative font unit such as %'s then they have no trouble at all....
The best CSS font size in my opinion is 1em -- The browser default: eg. what the user has set it to in there pref's. That's a hard and somewhat idealistic rule to follow though and I must admit to not always following my own advice ;)
Nick
In any case, it's not something that concerns webdesigners, because they should give control to the users, not take it away from them.
Always use relative "em" units for font size, not "px" or something else. It's maddening to visit sites with tiny fonts that you can't enlarge because the webdesigners used "px" units.
If you use "em" units, your text will display at the same size on IE5+, Mozilla/NS6+, Opera 5+ (not sure about previous versions)... I've tested my pages on Windows and Linux, and the font sizes stay the same. Of course this is not true for NS4 and IE4, but that's not exactly news.