I've seen guys on the FS tour using as small as 12cm fins!
Ps. I don't know if there's footage of it, but I've seen Gollito at Fuerteventura do up to quad-flakas and attempt more. Insanity!
I guess it's a similar principle to using a very short and soft freestyle board in snowboarding...
... turn on a dime, is very forgiving to catching the rail and easily over rotates. Would love to try a windsurf freestyle board...