The size of a font can be chosen separately but the typography determines how legible the font is, a good distinctive and legible font is Truesdell or Agfa Rotis. A good font should not call attention to itself, but should instead make the information being written about easier to read and understand.
A good font should have large features and a prime example to look for is the height of a lower case "x." A good font should also have a subtle and restrained design, for example the weight changes within the letters should be slight and the serifs if they are present at all, should be unobtrusive. Any font with both large features and a restrained design should be very legible and go almost unnoticed when used allowing the reader to focus on the words as written.