8 inch diameter is really small for 12 of those magnets. If you crowd the magnets you loose a large amount of winding space and you will have to use much thinner wire. You will also get more magnetic flux leakage but the main issue is loss of winding space.
Your quantity of magnet even on larger discs will only support a 10ft prop in low winds.
With your crowded magnets I don't think you can hope to use more than 8ft. The 8ft will be faster and you can use less turns, this will let you keep slightly thicker wire.
You will loose out on low wind capture by going from 10ft to 8ft but unless you are in a very low wind site I doubt that you can control it at 10ft unless you do something very drastic to the furling. Unless you have some overriding reason to use those discs I think you are making a lot of trouble for yourself.
8 magnet 6 coil works fine on 8" discs but I have that arrangement on a 6ft prop with mppt and it is at the low end of desirable efficiency for that application, it may just about take 7ft without mppt.
You save yourself the cost of 8 magnets and produce a slightly de-rated machine. If you haven't bought the magnets then I would go for 8 pole but I wouldn't risk it at over 7ft except in a very low wind area and with precise furling.
Flux
