RE: Best size fin for 7,3 and s-type 115?
No team member, but maybe I could help.
Since Deboichets are available at competitive prices, I really can't see any reason not to give them a try. The SL2 are great performers and their moulded construction will live so much longer than any comparable performance CNC fin. It should be a good choice for a speed oriented board like the S-type, and in lighter wind you should be able to do without the added control of a less competitive/more raked/swept fin.
As for size for a 7.3 freeride, I'd go for a 36; but note I have a bias towards smallish fins.