The first thing to keep in mind when thinking about the history and development of typography is that many early printers were not just printers, but typographers as well. The first independent typefounder was a French gentleman by the name of Claude Garamond . Although not the inventor of movable type, Garamond was the first to make type available to printers at an affordable price. Garamond based his type on the roman font of Griffo (a man commissioned by Manutius to develop an italic type for the Aldine classics).
Those are two different things right there. Font legibility gas already been studied at some length, although there is still much more to be done. I suggest you start with Sofie Beier’s excellent book on the topic, Reading Letters: Designing for Legibility, which has a great recent overview of research about type and legibility. Or just do a search on “font size legibility study” and read some of the abstracts of research. You will find that even within the usual range of text sizes, a difference of 16% or so is quite significant (such as the difference between 12 pt and 10 or 14 pt).
There are many facets to the expressive use of typography, and with those come many different techniques to help with visual aid and the graphic design. Spacing and kerning, size-specific spacing, x-height and vertical proportions, character variation, width, weight, and contrast,  are several techniques that are necessary to be taken into consideration when thinking about the appropriateness of specific typefaces or creating them. When placing two or more differing and/or contrasting fonts together, these techniques come into play for organizational strategies and demanding attractive qualities. For example, if the bulk of a title has a more unfamiliar or unusual font, simpler sans-serif fonts will help complement the title while attracting more attention to the piece as a whole.