According to the Trek site, the 6500 comes in the following sizes: 15.5, 17.5, 18.5, 19.5, 21.5"

The 17.5" and 18.5" frames are both called "medium". The 19.5" frame is called "large". At 5'9", you are a bit on the short side of medium height. Trek calls their 54cm road bikes "small".

You might read this: http://www.sheldonbrown.com/frame-sizing.html

But, ultimately, the only way you're really going to find out which bike fits you will be to try them out.