[https://acoup.blog/2019/06/07/collections-the-siege-of-gondor-part-v-just-flailing-about-flails/](The optimum is no chain at all).

A flail is a really bad weapon. The chain makes it difficult to control, puts you at great risk of hitting yourself, while not giving you any reach advantage. Real flails were medieval agricultural tools that were sometimes used as improvised weapons, but if you had access to an axe or spear, you would use that. If you have a big spiky ball of iron, it's much more effective to put it at the end of a rigid wooden staff and whack people with it that way; in other words, a mace is strictly better.

That said, real chain-based weapons do have their uses. The lkusarigama is made by attaching a sickle to a wooden handle with a long chain. It is used to entangle and disarm your opponent, at which point you can close in and slash them with the sickle end. Since it involves swinging a sickle on the end of a long chain, it would never be used in pitched battle lest you hit your comrades, and in any case spears are more useful when armies clash. However, kusarigamas were quite handy in one-on-one combat; since they were easy to conceal and could be disguised as agricultural tools, they were primarily used by ninjas and city guards

So to give an answer to your question, if you're going to use a chain-based weapon, the optimum length is long enough to completely wrap around somebody. And in that situation, you want a fairly light, small business end, not a big metal ball.