I don't know about you, but I don't read a screen nearly as closely to my face as I read a book. If, for example, you normally read a screen at ~2x magazine distance, it stands to reason that screen type should be around ~2x bigger than magazine type, no?

The best article I've seen on the subject is this one:

http://ia.net/blog/100e2r/

And maybe follow-up with this one:

http://ia.net/blog/responsive-typography-the-basics