How to fit a bike 101
OK, when you buy shoes you know to try on lots as makes and styles offer a different fit and feel, right? bikes are no different. Get a bad fit and you wo't ride and won't use the bike.
Use frame size as a reference like mtnbiker72 says. then sit on it. With your butt on the saddle and hands on the bars, you should feel like the bike was made just for you. Not too upright, not too far forward with too much weight on the bars. this is the "cockpit", get this part wrong, and you'll enjoy the bike as much as a poorly fitting pair of shoes (think: bike hanging in garage collecting dust and $ wasted).
You need to figure out your effective top tube length (ETT). Get the LBS to fit you, or find a way to figure it out yourself, perhaps from your last bike or a friend's?
Cheers, Jim