There'e are multiple schools of thought on this. Shorter cranks = less stress on joints and easier spinning. Longer cranks = more muscle use so larger gears can be used, shorter cranks = better cornering clearance, longer cranks = better for taller riders, shorter cra.....etc.

Everything from homemade 100mm cranks on recumbents to 200mm+ cranks for triathletes

http://www.google.com/search?hl=en&q...e+crank+length

Powercranks and a few other crank companies make adjustable cranks if you really want to spend the time working out what's the optimal length for you.

http://www.google.com/search?num=100...length&spell=1