Picking a font size for the web used to be really easy. 16 points and you’re done. You could feel confident that your content would be decently readable on PC or Mac, Netscape or IE. Man, have times changed.
Typography
What's the Best Font Size For the web? Well, It Depends… — Imarc
Accessibility is the process of removing barriers to meet people’s access needs. It goes hand in hand with usability, affordability, availability, and overall good design. This article focuses specifically on visual barriers in typography.
Jakob Nielsen's 10 general principles for interaction design. They are called "heuristics" because they are broad rules of thumb and not specific usability guidelines.