All this is here somewhere but I can't help you find it.
Most of this comes back to experience. You start with the power you want and that determines blade size. From blade size and chosen tsr you can calculate cut in speed.
You can get this cut in speed from any size of alternator but you need a bit of experience to know if you can get the power you want from a chosen set of magnets.
Look at what others are using for the size of machine you want, that will give you a good guide to the magnets you need to start with.
Take your chosen magnets as a starting point and calculate the turns needed for your cut in speed. Use the maximum size of wire you can for those turns and calculate the resistance. You can then calculate the current into a battery at different speeds and you will see if it will match your chosen blades. If the resistance is too high to get the load you want then try more or bigger magnets and calculate again until you get what you want.
Alternatively you can start with magnets and try combinations to see what the alternator can do and then choose blades to suit.
Flux
