Why do you need dimensions for a sail on a boat you don’t own? If you’re just tying to determine what a new mizzen might cost, should you decide to take the plunge, you could probably figure about $2000. A new main on a sloop that size might cost $4000, but it’s the mizzen on a ketch, so it should be a good bit less. Is the mizzen also gaff rigged? That might add to the cost as well: more area and more corners to finish.
If you really need them and can’t take actual measurements from the boat you may have to look for the plans somewhere. (MIT? Mystic Seaport?) They may be included here too: