RE: Kombat 97, max sail
Like Screamer et al indicates it will work well with 6.5 - even at 82 kilos. For max light wind performance with that sail you might need a slightly larger fin like Screamer says.
What happens when you use bigger sails on a board is that it will take more and more technique to get any additional performance out of it until you reach a point when adding more area will no longer return earlier planing, just worse handling. But again, 6.5 is probably on the safe side on a K97 for you.