Have someone at a bike shop size you and tell you what size you need. And I'm not talking about REI or any generic place like that. I have a friend who bought a bike that was WAY too big for her at REI. Lucky for her REI takes returns any time, no questions asked.

And when someone who knows what they're doing adjusts your seat, don't move it down so you can reach the ground while sitting in the saddle at a stop light. This same friend did this and it severely messed with her riding. You shouldn't be able to sit on the saddle with your leg on the ground.