Quote Originally Posted by VW Beetle
I recently bought my first mountain bike, and although nothing is actually uncomfortable, I am beginning to worry that I should have purchased the smaller size. In particular, if you are new to mountain biking, your roadie instincts about proper fit may be very misleading.

I am 5'7 with a ~31.5"-32" inseam (long legs, short torso). The two LBS guys helping me with my purchase disagreed about whether I was a better fit for a 15" or a 17". I went with a 17" WSD Rockhopper, which actually has the reach of the 15" men's model, but the height of a 17" model. At the time I purchased it, I thought the reach was very close (almost too close) and my greater concern was that I might be buying a bike that was too small. (My understanding is that Rockhoppers are known as relatively short reach bikes, in contrast to Gary Fisher and others known for their longer reach). I have 3" of standover clearance and 6.5" of visible seatpost. I am not uncomfortable in any way, but I do have this nagging suspicion that the 15" would be more chuckable and nimble, as others have aptly put it.

PS: I have most often read 3"-6" of standover clearance is ideal, with standover measured by standing astride the bike, lifting it by the seat and bars, and assessing how far from the ground the wheels are.
Sometimes you have to ride the bike awhile to get comfortable on it. You can also change out the stem length (shorten it) and pull the reach closer. I ride the 17" men's Spec. Stump and it fits me fine. I even have fairly short arms and I'm OK. I can't recall if I change out the stem. I might have. The 17 WSD s/b OK. If not, take it back to the shop and ask them to help you adjust the fit.

Now strangely my Fisher has a shorter reach but it is a slightly smaller bike (16.5") so maybe that's why.