As far as I know, there's no specific answer for this question.
That's because freestyle, as a core concept, is not standarized.
That's the reason why competitions here never asks for certain kind of materials or lenghts of nunchaku used by the contestants. Also that's the reason why you'll find different measures in different competitions from other organizations. Rules for WNA competitions are different to those rules from ASKA competitions, and so on.
It's free-style, after all.
In my humble opinion, if you want to find the ideal measures for freestyle nunchaku, I would say try different weights, lengths and materials so you can feel the difference between them and find which suits you most.
That's why you'll see many chuckers who makes their own nunchaku's instead of buying from stores, although there are some really good stores out there which you can always try.
Of course you could always keep in mind when trying different kind of nunchaku, that it shouldn't be too heavy for you to handle it, but also shouldn't be too light for you to control it. Try and find a balance
