Most studies of reading and typography have been done using paper documents, with higher-resolution type than computer screens currently display. Nonetheless, with today’s improved computer monitors and the benefits of anti-aliased type, we can draw some useful lessons from print reading studies:
Readers like large type more than most designers do. It’s not just the taste for graphic subtlety that drives the dichotomy: web designers are usually under great pressure to “cram in as much as possible,” and smaller type means more words per inch.
Generous leading (line spacing) is a key to legibility. Larger type helps, but the data suggests that a moderate type size—11 points—and a standard 13 points of leading yields the best balance of type size and overall reading comfort.
On the web type size is controllable through the web browser, as it should be for universal usability. Studies of web browser users also show, however, that the overwhelming majority of users never change the default type settings in their browsers, so choose a type size comfortable for sustained reading or risk losing many older members of your audience.