170 works well for me, 175 is fine too, and I have put down some real power/speed with 165mm cranks. I do modify my cadence slightly depending on crank length, but I don't think it matters too much. I'm 5'11" with a kind of short inseam for what its worth. I think the best for me is 170, or maybe 172.5, but I don't care much either way.