Cut in speed depends on the number of turns, flux per pole and frequency ( depends on number of poles and rotational speed).
The flux per pole is dependent on the magnet material, area and air gap in proportional; to magnet length.
You need to choose a cut in speed where there is enough power in the wind to supply the machine losses. If yo go too low the blades will not come out of stall and you will get nothing.
If you make your cut in too high you will not extract useful power that may be available in lower winds.
Typically a low loss alternator can be expected to cut in at about 7 mph, if you have significant iron or other loss you may do better to cut in at up to 10 mph.
Without clever things you can only track a useful band of wind speed so if you are in a particularly windy area you will do better with a higher cut in and higher maximum power. For a poor wind area you need to extract what you can in low winds and accept poor efficiency in high winds that happen infrequently.
Flux