Standard
spinnaker size is usually luff lengths = the boat's "I" measurement, and the girth is usually 1.8 times the "J" measurement.
Depending on which model/
rig N27 you have, your I measurement ranges from 33 - 37 feet, and the J measurement is 11.5 feet. You can do the math. If you are shopping for a used kite, anything close will likely do for you.