Oliver Reichenstein
[16 pixels] is not big. It’s the text size browsers display by default. It’s the text size browsers were intended to display… It looks big at first, but once you use it you quickly realize why all browser makers chose this as the default text size.Source: http://www.informationarchitects.jp/en/100e2r/
I have used 16pt body for my application - with testing, this was the most readable size without it being too large.
Typeface
I chose to use serif type for content because I wanted certain elements of my app' to feel like they'd come from a book. Thanks to Matthew Carter and Tom Rickner, there is a beautiful serif, Georgia, designed specifically for viewing on screen.
The typefaces you normally read have been designed to be typeset digitally and printed on paper. But as more people read more type off computer screens, they're unfortunately reading a type designed in another era for another medium. That's why type on screen can look pathetic (especially so-called "italics" on the Mac) and is often painful to read.Source: http://www.will-harris.com/verdana-georgia.htm
Luckily, a company with the deepest pockets in the computer world thought deeply enough to see what a problem this was and hired a type designer with years of experience designing legible type under difficult circumstances, including Bell Centennial for phone books, and ITC Charter for low-res printed output.