LOL !
They have to be just long enogh, but not too long!
Seriously, you are surely asking the wrong guy.
I think I can tell you some physical manifestations,
but I have very little understanding of how some guys can say
"Use a 4.3' dia with a TSR of 6.2" and be right on. It just never works out for me.
OK. The blind leading the blind. Here goes.
With the same TSR (Tip Speed Ratio) A long blade will start easier than a short blade. The trade off is the long blade turns at a slower RPM than the short blade.
Real example...
I have a stepper motor I had high hopes for. Put on a longer set of Zub-Woofer PVC blades, because they happened to be on the hub that fit the shaft.
Startup was no problem, but even in a good wind the RPMs were not fast enough to get up to charging voltage.
So I put on a shorter set and they worked great.
So I put on an even shorter set, but they were to short to get really started, because of the cogging.
And when they did get started, the RPMs were high enough to reach cut-in BEFORE there was enough power in the wind to make what the generator wanted to make.
So they wanted to turn faster, but the power to the battery was more than the power in the wind, so it almost acts like electric brakes.
That is my understanding of one kind of stalling.
The other kind of stalling is when the first blade to pass a spot leaves a large wake of turblence behind it, and it interferes with the next blade. At a certain RPM, the thing just can't go any faster because the air is swirling every which way, one blade to the next, keeping the blades from working like they were designed.
High TSR and lots of blades do this kind.
The standard 6 magnet 6 coil box fan conversion's BIG problem is the cogging is huge. To overcome the cogging, it takes a big slow blade, but then the big blade has a low RPM, so it takes a LOT of wind to reach RPMs for charging voltage.
So it ends up being a blade that should make 1000 watts (and tower and related strong enough to handle 1000W of wind) all for a generator that only can make 50 watts. Jerry's garbo-gen with 4'dia "Jerry Blades" makes what, like 1000 or 2000 watts?
I'm sure most people feel, by the time you have the big blades, strong tower, etc, needed to overcome the cogging, you may as well have something that makes some 'real' power.
That's why I thought I'd try it this way. Keeps everything small, cheap or even free, and easy.
It just worked much better than I expected !
(I have 20 failures for every success. No kidding. But I always do stuff that is kind of strange, and always small. Bigger is easier. Proven designs are easier. I wonder if this is a proven design? But I find this strange stuff much more gooder funnlier, and fun is the only reason I do it anyway. Hardly a place for a 20" windmill where I live anyway.)
I'll post a bad news (rust) here too.
G-