ALSO, Some electrical theory for you:
Power = Volts * amps.
Power = Volts ^2 (squared) / resistance of load.
Since you're using a DMM to measure your voltage output, and there's no additional load, the DMM's multi-MegaOhm impedance is the load. Divide by near-infiniuty, and you have nil (approaching zero) load slowing the motor.
This means that SPEED is the sole determiner of your output. Most windmills you see are designed to drive a hefty load, and are built as such, and that's not the contest criteria here.
Best ones I saw (so far) were all small, two and three blade devices, all well-balanced.
Uncle Fester, Maker & Fiction Science Writer
The Misadventures of the Electric Detention
The Revenge of the Electric Detention
The Curse of the Electric Detention
>> Three full-length adventures, 26 short stories and counting!