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
The sprint is a five-day process for answering critical business questions through design, prototyping, and testing ideas with customers. Developed at GV, it’s a “greatest hits” of business strategy, innovation, behavior science, design thinking, and more—packaged into a battle-tested process that any team can use.
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.