Short version:
longer cranks allow more low-speed torque, but can't be spun as fast and is considerably harder on your knees on long rides.
shorter cranks is the other way around.

You don't see a whole lot of variation among upright bikes, but it's not unusual for people with recumbents to run shorter-than-usual cranks: 150mm cranks won't raise any eyebrows in that crowd. Even fairly-tall people use them.

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It would seem to make sense to me that crank length should be somewhat proportional to leg length, but many studies have found not much difference in outputs. Overly-long cranks do tend to cause knee problems, and I wonder how many of these studies used durations long enough to detect that.
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