As a guide the relationship between Sail and fin size is
Times 5 add 3 !!!!
So for a 7 metre sail;;;; x 5 gives 35cm Adding 3 gives 38cm
7 metre sail (starting point) fin 38cm..
Works pretty much all disciplines !!! AS A GUIDE !!!!
Faster you go ; smaller the fin needed.. If you are going like stink on 7 metre on nice flat water you would get away with much smaller fin than marginal powered 7 metre >>
If you experience control probs;board trying to lift windward rail try smaller fin. If your mates are going upwind better than you ;try a bigger fin !!!
Experiment til you feel comfortable.