The stock set up for an 18/SX spinnaker would use an 11.5 foot pole, with the halyard 23.5 feet from from the mast base. For my set up I used a pole 12.5 feet long, as most people I talked to said the 11.5 is too short. The 12.5 worked great for me - no lee helm and it seems to open up the sail plan. Some have said you can go 13.5 max.
I can give you more info on how I set my boat up if you're interested.