Are you doing wave sailing only or do you need the faster crossover feel from the Kombat? If you want to add volume it's easier to get away with it once on a wave if you go for a board like the EVO (90). I think you would find the EVO 90 an amazing wave riding board for 5.7 conditions.
For flatwater, backside sailing and that type of thing a larger Kombat will work fine. But it can get a bit hectic on a good size (light wind) wave.