Since you are making a new sailcover from scratch, now is the time to modify it to a stack type if you don't have it now. Much easier to use.
Make the stitching so that the sun does not eat up the exposed thread wherever you can.
Also, yes; a bit looser than what you had allows needed air circulation.
If you leave a 4"- 5" flap to overlap the top closure of the cover, over the flaked sail, and add several velcro patches every foot or so along this top flap, or twist snaps or short ties, you won't have to fit a long zipper or pull on it to open or close it and will not have all that sun exposed stitching either.
If you line the inside of the new sailcover with another synthetic material cloth, it also protects the sail by preventing the UV rays from the sun that still manage to go through the sunbrella, just as you can still see some sunlight through the fibers on a bimini
top or awning.