fins letting go is called 'Spinout'. And while we all first want to change fins to fix spinout, it is usually sail tuning that is the culprit.
The main way of fixing spinout is to simply apply more down haul.
Then if that doesn't work, go to plan B, apply more down haul.
This of course assumes you have a reasonable quality fin.
once you have your sail tuned correctly then read this (they are assuming you know how to tune your sail already)-
http://www.iwavesolutions.com/lefebvre/Windsurfing/Articles/spinout.htmlthen try
www.blackprojectfins.com/there are charts there that say which size fin for which size sail that may make sure you are in the ballpark.
My 135L board is happy on a 42cm freeride fin.
As a PS: more downhaul usually moves draft forward, reduces back hand force, gives a more forward vector to the power of the sail and takes pressure off the fin which can cause cavitation. Hope that helps.
