The diamater of the hub takes effect on the precision of the ignition timing, bigger diameter allowes better setting,
but too big diameter will have too high linear velocity of the magnet and then the system might not get well
exitated by that; i.e. all components have to be "tuned" to each other. In general, maybe the best and even the cheapest
is to buy the whole set and instructions for it. By personal experience I can tell that the basic things are not big problem but
if you want to have good performance then secondary effects make sure that you might vaste the whole summer for
setting it properly, and still not get what you wanted i.e. it is as "simple" as playing golf ...