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@gbj_tester
Something like the Specialized BG fit system (available at Sigma Sport) will determine the maximum crank length you can use, based on range of movement at the hip. Below that, the available science indicates that there is no very good reason to choose one crank length over another, all that happens is that you trade distance moved at the pedal for frequency, i.e. pedalling at 150rpm on 175mm cranks uses the same muscle energy as pedalling 175rpm on 150mm cranks. This is why Sheldon Brown's Gain Ratio is so useful, because it tells you exactly how much you'll need to lower your gearing to suit shorter cranks, or vice versa. Since all this seemed counter to old school wisdom, I actually tried it, and it's true; I went exactly the same speed over the same 10 mile course using 140mm cranks and 72" gear as I did on 175mm cranks and 90" gear. People have done tests with cranks from 100mm to 200mm and found no clear difference in biomechanical efficiency across even that wide range. There is some evidence suggesting that cyclists fatigue less at a certain cadence, and that this cadence tends to be slightly higher in trained subjects, which possibly indicates that you should shorten your cranks as you get fitter, but the difference between the extremes of common road/track cranks from 165mm to 175mm is so trivial that it really isn't worth worrying about. It is also the case that, despite claims by incredibly obsessive bike nerds, nobody can actually tell without looking if you swap one or both of their cranks for something 2.5mm different in length.