What size mountain bike?

[Irulan]

First don't get locked into WSD, a small men's frame might be just fine.
Second, if you are used to riding a road bike, most likely a properly fit mountain bike is going to feel shorter in the cockpit than what you are accustomed to. I made the mistake of fitting my first mtb too long based on what I was used to, and let's just say it was a mistake that held me back for a few years.

here's what my tips page says about WSD:

What about WSD (women specific design)? It's a great concept, geometry and frames just for women. This concept is for a specific body type: women who have a short torso and long legs. I ride a men's 15" Kona which fits me beautifully. I have a longer torso. Don't get locked into WSD, but go ahead and try it and see if it's right for you. For some women, it's exactly what they need. Try lots of bikes until you feel the one that's right for you. Some things you can change out on your bike are smaller brake levers, shorter cranks, and more narrow handlebars.

If your LBS (Local Bike Shop) is insisting that you try WSD, or only WSD, and treating you in a condescending manner - "hey little lady, we know what you need..." Run, do not walk, to a different shop. You deserve better than that. Try lots of different bikes.

ride a LOT of bikes. Fit is the most important thing, over brand, over suspension, over wsd etc.

Irulan