Trek 1000, two mtbs and working on a fixie for commuting.
Ride it and find out. People are different and guidelines for sizing are estimates at best. I am 6' 1" and I ride a 53 when I *should* ride a 58+. I feel all sorts of pain on a bigger bike, but for mtbs I can't ride something so similarly small - bodies are weird like that.
If you want a really good estimate to start off with, go to competitivecyclist.com, click on the box for "road" and then use the fit calculator link in the bottom right.