Well, the easy answer is that I went and measured mine to class rules and found that although for a different class, it was actually very close to the official size. I even think it would be class legal, but the class measurer might find I was off by an inch. (I don't race in DSA sanctioned events, so that is moot - what we have here is a non-handicap race where, in principle, I could add any kind of sail - some people here have flown spinnakers on a Laser).
The most up-to-date class rules are posted on the
DSA site. (Here's an excerpt from the 2012 version which I used when I measured my spinnaker).
8.14. SPINNAKER: The spinnaker may be of any woven material of weight no less than 1/2 oz. per yard. Only one spinnaker may be aboard while racing. No headboards or oversize grommets are permitted.
8.15. The size of the spinnaker shall not exceed any of the following dimensions: Head to clews: 15 feet 3 inches,
Head to mid-foot: 16 feet 0 inches, Mid-foot to clews: 6 feet 0 inches.
To measure head to mid-foot and mid-foot to clews, fold the sail vertically by bringing the clews together. All measurements shall be made in a straight line, with wrinkles removed and no tension on the cloth past removing wrinkles.
I don't think those measurements have changed since then, but I know other details of the class rules have been adjusted.