mmmmmm lots of more info required....
It is mainly determined on what sort of board you have, width of board 300mm from the back. Type of sailing you want to do, upwind,downwind etc.
Sail size is only a small part of determining fin size.
The leverage you can develop on the fin will play an important role in what size to use.
If you are heavy and have a wide board then you sail with a bigger fin. If you follow the program here.
On my small board I use a 36cm venom and a 40 cm venom on my big board for the same sail size.......
more info needed!