johnyfav - 11:27 am on Feb 3, 2010 (gmt 0)
I've often heard designers say that typography is the web.
I also believe that if you hold up a book to the screen and sit the usually distance to said screen and book you get a fair idea of what sort of size text you should have.
I go with 16px/1em. I set all my sizes in em so that they are scalable if the user requires via the browser.
It's not just the font size but correct line height and also how you apply margin and padding to elements so that vertcial rhythm is achieved making reading easier.
I also think there is an optimal reading width - To short or too long for the paragraph to fit in to and the text isn't as easy to read.