RE: Putting an NP Excess 6.9 on a Kombat 97?
I agree with JM, but its worth adding that how big sail you can put on a baord is not only a matter of "performance" but also depends on how much "imbalance" when just planing you're willing to live with and that your technique can compensate for.
In any case, on a given board, in the end you come to a point where more sail area will simply not let you plane any earlier. My GUESS is that a 6.7 will be at least as effective as a 6.9 on a Kombat 97 for most people. Sails charactersistics wil matter to btw. Lighter, flatter and "springier" will let you go further than loose leach, deep profile sails because you can be more effective with pumping in the crucial stage when just getting onto a plane.