Rotor size for 8 pole 3/4 magnets?

I'm working on a project to generate power to charge phones and tablets in remote parts of Kenya for medical clinic work. We have a low cost solar solution that plugs into 6 volt deep cycle batteries that can keep the phones running for days. BTW cellular service is great but no local power in many cases.

What we are working on now is wind power for the 1/2 year that there is limited sun but lots of wind. The goal is to charge 6 or 12 volt batteries. We also want to keep the cost as low as possible and use common parts like bicycle bottom brackets and wheel hubs that have disc breaks.
Here is one of the builds with that makes about 1.5 volts hand spun. http://youtu.be/9AILyg_BzKo

It seems that the rotor is to small 3", so we have some 6.5" discs ready to go and 20 - 3/4" magnets. The blade radius is 20".
Is there a chart somewhere that could help me figure out a good idea for the generator size? I would like to go with a dual magnet setup of 8 magnets/rotor (16) and 6 coils. Does this even sound realistic?