I don't have Dan's book and the Otherpower web site seems to be being updatede at present so I can't get at the basic Otherpower 10ft details.
The thing with the coils is getting the number of turns right for correct cut in at your chosen battery voltage. Set up your magnet rotors and try a test coil of a size that will fit in the final layout. Pick a number of turns using the otherpower instructions as a guide ( nearest specified wire size). Run the machine at known speed. Bearing in mind that your coil is 1/3 of one phase you can work out what voltage the winding would give with those coils at cut in speed ( 150rpm ish).
You can then scale the numnber of turns to suit what you need for chosen battery voltage.
Remember that the ac volts you measure is 1/3 of a phase, so measure the coil, multiply its volts by 3 to get phase voltage. Then multiply by 1.7 to get line voltage and then by 1.4 to get mean dc voltage. If you are turning it at 150 rpm then this should equal battery voltage. If not you have to include the speed scaling factor.
You can do the test coil with any size of wire, but if you use your #15 you will be sure thast your coils will fit.
I suggest you start with a test coil about 1/2" thick or a little less so that when moulded it will give a 1/2" thick stator. Try your magnet rotors about 3/4" apart and if your coil comes near you can play with the magnet gap a bit either way.
The main thing is correct cut in, take the resistance as it comes, it will determine stator heating but if you get fully wound coils in a sdtator 1/2" thick with near enough cut in it will be ok.
If you can post the otherpower winding details for various voltages and wire size I may be able to help a bit more.
Flux