Make the spinnaker pole come to exactly 1 metre in from of the front of the hulls. Will give plenty of spinnaker and keep it controlable throughout a large range of conditions and the boat will pretty well balance. Don't go for "the largest spinnaker that you can fit on the boat" as you would find that you will have area but you won't be able to set it in all but a few specific conditions, and it is better to have a smaller spinnaker that sets in light airs than a giant that won't fill. In heavy airs the smaller and more controlable the kite the better you perform.
Darryl