I'm not even all that outside of "average," but I live in a region that is heavily Dutch (the tallest ethnic group on the planet), so my 5'3.5" puts me solidly in the "short" category. As a result I have a hard time finding bikes to test ride, especially since our LBS of choice is a small shop. I have come to depend upon our LBS friend to determine in advance whether a bike will be a good fit for me--and so far he's done a pretty good job. My CX bike is actually the only one that was actually on the shop floor in advance. For months he was "suggesting" that bike to me. And it's the best fitting bike of any I've ridden.
I'm now in the market for a 29er and have it narrowed down to 2 bikes. I'm pretty much going to make the final decision based upon what Mike recommends, most likely. We've ridden hundreds (thousands?) of miles together, so he has a pretty good idea what will work for me--probably better than I could figure out on my own, since I'm still a noob.