Hello Bruce and Sparky,
Since the centerboard force opposes the side force from the sail, centerboard area is directly related to sail area. If the I17 had the same sail area as the I20, then the same boards would be right for both boats. If the I17 sail area is, for example, 20% less than the I20 sail area, then 20% less board area is required to sail towindward on the I17.
Also on other points of sail where the boat goes faster than to windward, board area can be reduced because the lift capability of the board area increases as the square of the boat speed due to the dynamic pressure of the water. For example, if a boat sails to windward at 10mph but can reach at 20mph, then at 20mph only 1/4th as much board area is needed as when sailing to windward at 10mph.
Bill