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
Knowledge
User Interviews: How, When, and Why to Conduct Them — NN/g
User interviews have become a popular technique for getting user feedback, mainly because they are fast and easy. Use them to learn about users’ perceptions of your design, not about its usability.
Interviews
User Interviews: How, When, and Why to Conduct Them — NN/g
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.