jjboods:

I'm 5'10" with a 32" inseam (pants-wearing measurement, that is).
My Trek 7000 is a 20-inch, and I can't imagine riding a bigger bike comfortably. I probably could have ridden a 17.5-inch; I did test ride one briefly before going with the 20-inch. But devildogmech is right - any LBS should make sure you ride more than one size and find the right one for your body size and shape.