Initially it is more difficult to create a good layout with a big font size, but that difficulty will help you design a simpler, clearer site. Cramming a site with information isn’t difficult, but making it simple and easy-to-use is. At first, you’ll be shocked how big the default text is. But after a day, you won’t want to see anything smaller than 100% font-size for the main text. It looks big at first, but once you use it you quickly realize why all browser makers chose this as the default text size.
Information Architects, The 100% Easy-2-Read Standard