nz6666, In my experience, typically a shim (under your right foot/the shorter limb) is better than adjusting the length of the crank arm. Using a shorter crank arm on the left will utilise less of your limb range of motion and could result in a possible decrease in power.
If possible, try and accurately measure the LLD (limb length discrepancy) with a physiotherapist or orthotist that specialises in bike fittings.
Luke