RE: Larger Fin for my F155
That depends to the greatest extent on how wide youir board is.
A board with a narrower tail width (and less footstrap offset) will not benefit from a really long span fin in the same way as a board with a really wide tail width.
Plus, at some point, without the requisite footstrap offset, you won't be able to control the attitude of the fin very well.
It's a balance thing between the rig size, fin size, footstrap offset, and tail width.
To simply say that if you get a 9.6 m2 rig, you need a ?? cm fin doesn't nearly take into consideration all the factors here.
Board volume, tail width, footstrap offset, and volume distribution are the board factors.
Rig type (race, free race, or recreational can be another factor.
And, the conditions you intend to sail it all in.
Hopes this helps,