Sizing is a complicated issue, and some of it depends on the type of bike, and some of it is based on opinion. Furthermore, conventional wisdom has changed recently, now with the advent of the "compact frame" design that has a top tube that slopes down from front to back.

A wonderful fellow named Sheldon Brown wrote more stuff about bikes than perhaps anyone else, and he wrote it in a very accessible and useful way. Sadly, he died a year and a half ago, well before his time, but the writings he left behind are a huge wealth. I suggest you take advantage of it. It's at http://www.sheldonbrown.com

His article on sizing -- as you knew there would be -- is http://www.sheldonbrown.com/frame-sizing.html