Sails are made by a number of different sailmakers, so there is no book you can go to (or web site) that will tell you what size batten you have. It's like going out on line and asking someone what size shoe you wear. So, as suggested, measure it and buy it from your favorite sailboat store. Or, contact the sailmaker, who might have your answer if it was custom made. If it came with the boat out of the box, then measure twice and cut once.