Hi Mike.
VAWT's are bit harder to plan for, because less people build them, and you've got less reference examples.
However, looking over the Hugh Piggot design for a 16 magnets/12 coils HAWT, I see the alternator has a cut-in speed of 120rpm, for 12V. It uses 25 turns of #13 AWG wire, 2 in hand. So, to make it a 24V, we'd simply use 50 turns of #13 AWG wire, 1 in hand. (Same thing exept instead of winding the copper two in hand, you wind it one in hand).
Now, based on the calculations of RP (previous poster), your gonna get around 84rpm in a 10mph wind. So, 120rpm cut-in is looking a bit high. I'd double the number of turns again so that you'd have reached cut-in and would be producing some amps at 10mph. So, the coils would become 100 turns of #16 wire. Now, if we look at the resistance of that, 2.4 ohms, that's quite a lot, and the plan is to put the resistance out of the stator, so you can play around with it. So, I'd keep 100 turns, to keep the approximate cut-in, but I'd go with thicker wire. Probably #14. It will give you bigger coils, but it will drop the resistance way down, and you'll be able to add some more outside of the stator. (which you can then use the heat for other things, fryed egg anyone? hehe).
I'd look at this table http://www.amasci.com/tesla/wire1.txt as it gives quite nice comparative values for different wire thickness. The thicker the better, since the plan is to have a stator with a low resistance, and add some more outside to keep the current down.
Option #2 would be to use the bigger 3"x1.5"x0.75" magnets. I saw a post about somebody having bought some, and he said that although they have 3.375 times the magnetic material, they weren't close to 3X the cost, so, you might be able to get yourself a quite powerful alternator with those. They're larger, so you could have quite bigger coils with quite thicker wire, and have more flux pass through them. Since you don't have the expense of a tall tower, that's probably the way I'd go, to invest my money. You might have to do a bit more trial and error, but If your looking for a cut-in speed for say 70rpm, you could look at the 16/12 17' HAWT DanB built, and probably estimate a starting point from there.
Anyways, hope it helps,