RE: fin for isonic 105 2006
I am 78 kgs and I use a Tectonics Goldwing 34 with my iSonic 105 and my 6.6m. It works very well upwind, downwind or crosswind . I have tried smaller fins (33 and 32 cm, good ones, but not Tectonics), and there were a number of spinouts. If I was 87 kgs and on a 7.0m, I would probably use a 36 cm fin.
Good luck with your choice!