Your best bet is to talk to a sailmaker. He will have to come and look at the boat and measure things. Mizzen staysails can vary a good bit, depending upon the owner’s desires and the boat’s design, so the cut and size are not set in stone, even if the Naval Architect drew a sailplan that included one. Where the sail is tacked to the deck can also change how it works. Mizzen staysails, done right, can add a half knot to boat speed. Getting them cut and set so as to do that is not necessarily easy.