The studies I've seen about readability and legibility tend to focus on a specific set of metrics: size, not just the point size, but things like the size of the lower case letters as a proportion of the overall letter height, and line length. People simply can't read really small type set in really long lines.
Michael BierutNot everything is design. But design is about everything. So do yourself a favor: be ready for anything.
Michael Bierut