This is contentious:
the theory is leverage, so the longer the cranks, the more leverage? makes sense in that if you had cranks twice as long as you currently ride you could exert at least twice the power, may be more, and using physics you could prove that.
however in cycling you have to keep them turning and those big cranks would be impossible to turn, therefore of little or no use.
So it comes down to the size that suits not only your physical size, but your riding style. if you big, you have longer cranks than a shorter person, thats a fact due to natural leverage efficiencies of your physique. However riding style comes into this too. if you like to grind, mash, big gears slowly, then a longer crank is going to give you more leverage, that's a fact too.
if on the other hand you like to spin, you will be like a high revving engine, and the more you can spin (revolutions per minute) then the more power you can produce, and that's a fact too.
Mike has experimented quite a bit with crank lengths and changed to longer cranks, thinking he will have more leverage and therefore with every push, develop more power, and then go faster. He has found (and he may wish to correct me) that when you go to long cranks, you simply can not spin the cranks fast enough to be of any use. Remember this has to do with the style of riding that you Naturally prefer, (and you can train your self to spin faster) he tried 175's which felt like huge circles to him and thus No good to get the speed up, despite each pedal stroke having more leverage. Also 172.5's and 170's. On the smaller size he is able to spin up fast (accelerate) faster, and spinning faster is more beneficial to power.
Studies show that to produce the power, you spin! so the biggest crank set that you can spin fast, may be the best? unless you don't like to spin. Another foot note is, that long cranks can hurt your knees. I have 2 bikes, one with 175mm cranks and the latest has 172.5's. When i discovered the discrepancy, I thought I want longer cranks, like the other, I can spin that fast too? so went back to the shop to talk about changing them. I was convinced by the very informed (now a pro coach) that I should be on 172.5's. I persevered with them and well, for the first time, last year after riding Taupo in my best time (17mins faster) my knees were not sore at the 100k mark. I am 5'10-11"
That's why it is all so contentious and I will list the recommended crank lengths (according to Chris Carmichael) for the frame size, and of course it is all a matter of what bike fits you.
Frame Size up to 54cm crank length should be 167.5-170mm,
Frame Size 55-57cm crank length should be 170-17205mm,
Frame Size 57-61cm Crank length should be 172.5-175mm
Frame size above 61cm crank length should be 175mm.
I have seen cranks of 180mm ridden by a guy who was 170cm in height, and he was fast, likes to grind those gears, :)