Hello Claus, look at
http://www.select-hydrofoils.com/en/...ump-freestyle/ They recommend fins between 27 and 29cm for ~90 liters boards and 4.2 to 5.4 sails. Never tried their Bump & Jump but I use fins of that size or even bigger for hi-wind freeride (and I'm quite light). Another option is the SRX FreeSlam 28/30 which will give you a lot of grip.