I don't know where you are located, but the 5.5sl guys in Florida have gotten together and agreed on a maximum limit of 230 sq ft. Read all about how they set up the boats at 1design.net

350 or so is about the New England class size for the Nacra 6.O, and while it ain't one of Randy Smyth's Worrell 450-footers, it's all the power the boat can handle most of the time. Plus, another advantage of a smaller chute is that you can carry it beam reaching higher than a larger sail. Finally, the used sail will let you get your feet wet (so to speak) with spinnaker cats. When you have the bugs worked out of your rig, have someone (Randy or other) make you one, or buy one off the shelf.
sail fast