Not an expert, but logically the shorter chain makes it easier to handle and relatively more precise.
A longer chain however, allows for a greater swing, resulting in a heavier impact.
That however can also be achieved using a heavier ball on the flail. But that makes it heavier to lift and carry.
Using multiple balls however seem counterproductive, as it will make the flail harder to use compared to the impact you can achieve.
I suppose the idea is to make it harder to defend against, but if the defender has a shield, I think multiple balls are utterly inefficient.

So as far as I can tell there is no single optimal balance. It depends on the persons strength and agility, and I suppose it also depends on what type of enemy it should be used against. For a heavily armored opponent, a longer chain will be better to smash hard against the heavy armor, and the armor will make the opponent relatively slower to avoid attacks.