A 3ft machine decently sited will do it easily. Unfortunately not all sites where fencers are used are exposed to decent wind. In a poor site a 4ft machine may have a better chance.
You can probably manage without furling but the simplification is negligible and without it you have more noise, speed and a far higher tower loading in high winds.
Why use other than 3 phase, single phase simplifies nothing and has nothing going for it, I say the same about delta, no virtues at all.
HD magnets will do it, but as usual it is hard work to save a tiny cost on more convenient magnets. If you don't use HD magnets stick to reasonable ones, tiny magnets even for a small machine are never worth it. That is probably why Hugh uses the 2 x 1 x 1/" magnets on his 4ft machine, they are common and cheap.
I don't know about HD bearings, I assume they are good for speed in a perfect environment, I doubt that they will last many minutes for this. I tend to underestimate this sort of problem as anyone with a lathe can sort out hubs and bearing housings quicker and easier than combing the district for something that may just do.
If you want to go the non furling route then look at using too many blades with too much pitch so that it becomes self protecting in high wind. You need a much lower speed alternator but it need only produce a small current. ( invent yourself a Marlec, that was built for fences originally).
Flux
