A lot of people are cutting the fin at not much more than a hands width - 18 cm would be max. Taking their freestyle or small wave fin, and cutting it. I used a Dremel tool. You then need to sand / fair the leading edge and tip. in Bonaire, a lot of the kids practice finless, too. I guess the basic point is that it doesn't matter a lot, just cut it short. Weed fins work good (after they're cut), probably because cutting it doesn't really screw up the shape of the leading edge at the tip.