Yes, you hit the hammer on the nail; which comes first, the chicken or the egg.
Basically in my case it was trial & error on paper & cad-screen; draw a sketch of rotor; enough room for coils (but what size is a coil?)? But how many windings do I need, and what diameter, and... Redraw rotor with different size;
Anyway in the end I needed double the amount of turns/coil (130 instead of 70) with my too small NdFeB-magnets with too large air-gap; this meant using thinner wire, higher internal resistance, more losses in the stator, etc. But the thing is expected to only generate about 10W (.8m dia prop), 50-60W absolute maximum. It'll be used on the boat anyway. (Actually, I'm thinking about building a PMG to couple to the propellor-axle of the boat; when sailing, the propellor freewheels; it should be possible to extract some energy from it, even despite the low (100RPM?) RPMs of the prop). I have seen outboard generators like this before, so the idea has been tried before.
A windgenny is basically one big compromise, like any engineering project. I liked designing my own, even though I had never built a genny according to someone else's plans; forced me to do a lot of reading & learning in a short time. Made the odd error or two, but my genny still works (though it could have been better).
My advice: consider your first genny a learning experience, from which you will learn from the mistakes you make. (even my 2nd one was a learning experience). Now with the 3d one, I feel I'm ready for the real works. But the first two have been great fun building!
Remember: 'things become important to you not because of what they are, but because of the time and effort you invested in them' (Saint-Exupéry)
Peter,
The Netherlands.