Crank length really doesn't have an impact on power production. What it does impact, as it has been pointed out in a couple of spots earlier, is your aero position and your foot speed.
Shorter cranks open the hip angle more than longer cranks, for a given position. By moving to a shorter crank, one can lower the bars to get more aero (now, this is another discussion), while still retaining the original hip angle. (Dropping the bars decreases hip angle with the same crank; dropping the bars, while shortening the crank can maintain a consistent hip angle)
The other variable is foot speed, which is different from cadence. Some riders will move to shorter cranks to decrease foot speed and find it more comfortable/effective/efficient, etc.
Me, I only adjust crank length to match track construction--steep tracks, shorter cranks for clearance; road, normal crank length. FWIW, I'm 5'7" and have found that 175s work great for me, even when I fit an oversize chainring.
I also have found through a good amount of field testing that crank length can vary pretty significantly without have a negative impact on my power. The different cranks lengths requiring varying degrees of adaptation, but nothing major. I found that the same was true with non-round chainrings, too.
Don: Cycling Tech >