Interesting point. You certainly need a lot of magnet to guarantee the thing to stop on short circuit. For Dan the stalled operation is very peaceful and tame and has a lot going for it for battery charging, where the peak power in high winds is probably embarrassing.
Dave can get away with a lot less magnet as the voltage is rising with wind speed, thinner wire will still give an adequate rating up to the point where he needs to limit speed for noise and peace of mind. If he was to adopt Dan's size of alternator it would be even more efficient and probably if properly loaded with a cube law controller the 6 to 10kW figure would be realistic and it would still stop with a shorting switch.
Probably with no cube law controller the bigger alternator would make matching easier in high winds but it may worsen start up at the other end. It would allow short circuit braking.
Provided the alternator is powerful enough I think short circuit braking is ok. I have never done anything larger than 5ft without some alternative way of stopping it, but my things tend to be near civilisation and a run away machine would be a major embarrassment. I have always used the tail to stop it rather than a mechanical brake. As long as the connecting cables don't fail I think a brake switch should be ok with modern alternators, I started with wound field machines and in those days there were far too many failure modes to not have an alternative.
Flux