Five Simple Steps to Better Typography: Hierarchy—Size
http://www.markboulton.co.uk/journal/comments/five_simple_steps_to_better_typography_part_4/

Boulton, Mark
Mark Boulton
2005

Abstract:

Typographic hierarchy is how different faces, weights and sizes of typefaces structure a document. Some of these hierarchical devices are well-established conventions, such as cross heads and folios, so I'm not going to touch on them in this post. To keep it simple I'm going to concentrate on two things - size and weight. The first of which is size.