You can give them the IJPE info from the site for NOW. But when it comes time to order anything, make sure the sailmaker steps on the boat and measures for themself. If you do it and something doesn't fit, it's your loss. If they do it, they fix it for free. Measuring is not hard, but if it's your first time, let a pro do it, it's not something you want to learn through trial and error.
Off topic - The Venture 21 is a fun boat, I learned how to sail on one. Just inspect the pivot bolt and washers that the keel swings on. They leak and sink a boat rather quickly.
Merit 25 # 764 "Audrey"