Depends big time on a couple of things: the boom length, and your sail itself. On my '79 Oday 23, my boom is 9 feet long. Earlier models had a shorter boom. However, I bought new sails a few years ago; the old main sail cover didn't even come close to fitting over the new sail. It was plenty long enough, but when furled, the new sail was much bulkier than the old. The old cover, which was easy to fasten below the boom over the old sail, barely covered the sides of the new sail when draped across the boom. And my new main was the same "size" as the old. In short, anyone can give you the measurements of their main cover, but chances are it will be too large or too small.
My advice: invest in a seamstress' cloth tape measure (about $2 in the supermarket). Furl your main on the boom as you usually do, and measure at the gooseneck and two or three stations along the boom.
Give those measurements to your sailmaker to get a cover that fits. My sailmaker insisted on seeing and measuring the new sail on the boom before he would make me my new sailcover. I'm glad he did.