In my flawed understanding, crank length has more to do with leg length than anything, a second property of concern is pedal strike/toe overlap, so if I had short short short legs, I might want a 160 or 165 crankset, same deal for if I had a very low bottom bracket on a fixed gear and wanted to minimize pedal strike in turns (since you can't coast the pedals in a fixed position on a fixie when turning). My understanding also is 170-175 is fairly average crank lengths. Someone will be along shortly to tell you how wrong I am though and give a better answer I am sure.