I typically thing the sides should be 16 inches high or lower, but the flat floor is 6 inches above the hull.

With 18 inch sides, I have 12 inches from the cockpit sole to the sheerline, so I can put my seats at 10 inches above the floor for comfort and still have a little space under the gunnels.

If I rip the plywood into 24 inch pieces, I can use the entire sheet: The sides become sides and the excess becomes decking - PERFECT!