Bruce,
This is a question that only the I17 designer can answer quickly. If I tried to answer it, I would have to say, " well if you assume this, bla...bla...bla..., but if you assume that, then bla...bla...bla. and I don't know the this or that because I did not design the boat. The proper size boards and rudders for the boat depends on how the designer loaded them up. For a given sailplan board area plus rudder area equals a constant. If you load the boards more then the rudders can be smaller. If you increase the load on the rudders, then they should go up in size and the and the board area can come down. The best answer is check with the boat's designer.
Bill