J. Crew has a measurement for bathing suits here:
http://www.jcrew.com/help.snip.....jhtml#swim
Click: "Find if you have a long torso" and it will tell you how to measure. It's probably only for their brand, but it might give you a starting point.
I think having a long/short torso has a lot to do with how proportional it is to the rest of your body. For example, I am 63.5 inches tall, and my inseam is 29. I always have to wear short length pants (and they're still too long), when some girls who are my height have a longer inseam, because their legs are longer. I think I probably have an "average" length torso, but in proportion to my legs, it seems much longer than it probably really is.
It's good that you ask this, because it's important to find out, and dress so that your body looks more in proportion. I wonder if there is some "golden ratio" that is considered ideal, like, "Your legs should account for 1/2 your body, torso should be 3/4 the length of your legs, etc."