the Hushed Casket

Disclaimer: I'm not an expert, and I'm typing this during lunch. There is no fact checking, and no spell checking. WYSIWYG

19" usually refers to the length of the seat tube. The geometry of the frame can effect the stand-over clearance, so that alone isn't a guarantee of a fit.

As far as sizing goes, I typically have 2 concerns which are kind of inter-related. You want to have a frame which is long enough to create a "cockpit" large enough to accomodate your torso. You can extend this "cockpit" by purchasing a longer stem (thingy your handlebars bolt onto), but the further forward you shift your weight over the front wheel, the more likely you are to go flying over handlebars when you hit an obstacle on a downhill section. You can also get seat posts that extend backwards, but if you shift your weight too far backwards you'll start having problems keeping the front wheel on the ground when you climb. Also, if you let your center of mass shift too far back you can start losing pedaling efficiency.

So...a frame that is long enough to accomodate your torso and keep your weight from being a bigger issue than it has to be when you are on significantly off-level rocky terrain.

Buuuuttt. Usually the longer the frame, the taller the frame. You don't want to be wracking you nads on the top-tube when your feet are off of the pedals. A lot of frames now adays are manufactured without a straight top tube, so this could be a non issue. Depends on the bike.

You also need to be able to raise the seat high enough. A lot of noobs seriously underestimate how tall that is. Try this. Squat 1/3 of the way to the floor and walk around...now stand up. Which is easier on your muscles?

The local bike shops should all be able to help with sizing.

I agree with most of JayWHY's advise. I will say that my pedals are SPD's and I like them alot. I don't ride in the mud much though, so YMMV. I will disagree with him a bit on the full suspension issue, primarily because of the local geography. Once you get off of the top of mountain the bike trails are very rocky. You may spend a significant portion of a ride on trails composed of fist sized rocks. There are lots of rock-gardens. We don't have much smooth single-track. Most of the serious riders (even the XC guys) eventually end up with full suspension frames and save their hardtails for fireroads and trips.

However, you get what you pay for, so if your budget is less than around $1200, don't bother looking for a full suspension bike. Cheaper full suspension bikes often feature designs that add a lot of weight and absorb your pedalling energy. Get a hard tail instead. My bike is a hardtail, but when it comes time to replace it, I'll be coughing up the extra dough for a nice FS.

gtg

- Brick