I can't for the life of me remember the general rule as to how long spinnaker sheets should be? Was it 1 1/2 times the LOA or is it twice?
Make them as long as they need to be.
How long they are
is a function of how your boat is set up with turning blocks, cleats, etc. and the sheeting system you are using. You won't know until you get out there, pop the kite and see how much you need.