The answer is yes but no.... If you are using an old style Kettering type ignition system, and the plans for one are here on the web somewhere, then yes, the diameter of the "ring holding the magnet" has an effect. With that system, there is a dwell time to consider, and it is entirely possible depending on the diameter of your "ring", that you may have to even use more than one magnet to get a proper dwell.

However, all modern systems are based on a CDI design, and you use only one magnet and the diameter of the flywheel or magnet ring so to speak is unimportant.

AV8TOR