You don't want to use different length cranks, because the pedal circumference will be different on each side. Instead you want to use the same length crank, but you either need spacers on your short leg or your longer leg needs a custom pedal drop extender that lengthens the reach. Think about it, it will have the same pedal circumference on both sides.

Speedplay pedals offer an reasonably priced way to add spacers between the shoe and pedal
http://www.speedplay.com/index.cfm?f...home.zeroshims

or check out the axle extenders here:
http://www.hscycle.com/Pages/cantsandwedges.html
these are expensive tho.