I know that designers love using small-ish font sizes. It makes the text feel more sleek, and it devotes less room to copy and more room to graphics. And most designers treat copy as just a block of text anyways, so they don’t need to read the text.
Content-producers (particularly inexperienced ones) tend to love small text, too, because they feel like they can write more copy to include on the site.
But from a usability standpoint, I really need the text to be much larger than that – really around 16-18 px. First of all, I don’t want to have to hold my face 6 inches from the computer screen to read it. Secondly, an average column of text should have somewhere around 45-75 characters in it – just ask Trent Walton.
But using larger typeface also helps in a couple of other (pragmatic) ways. Namely, it forces content-producers to really focus their message. Less copy = better copy.
I could go on, but it’s a lazy Sunday afternoon, and Smashing Magazine already did a pretty good write-up on why you should use bigger font sizes on your website. You can read it here.