How to Match Type Size to Readership
http://desktoppub.about.com/cs/finetypography/ht/type_size.htm

Bear, Jacci Howard
About.com

Abstract:

The appropriate type size for a publication depends on many factors but there are some general type size guidelines to follow to insure readability for the main audience of your publication. These are not hard-and-fast rules. The more you know about your readership, the better type size choices you will make.