It is irritating to have a bag that is too small,
I don''t think this is too tough. I would fold the sail the way that I always do, stand it up on a piece of paper or cardboard, draw a circle three or four inches out from the sail all the way around, measure how high the folded partially rolled sail is while it is standing there. Cut out the circle pattern, & lay it on the bag material allow another 1/2 inch all the way around for stiching & cut it out. Cut out a piece of material long enough in one direction to go around the circle, plus one inch, that gives 1/2 inch on each end for stitching. The other way cut it six or eight inches more than the measurement of the sail standing, while it was folded and partially rolled. On this last dimension allow another one and a half inches for stitching at the bottom and the gather line tunnel at the top. Sew in the gather line tunnel leaving it cut back 1 1/2 inches on each end of what will be the inside. Sew the large squarish thing to the round thing, sew up what was the long squarish piece, where the two ends meet. And; we have a bag.
Ops! I forgot to put in the gather cord (or draw string).
If I were doing this at home on the living room floor I think I would push the couch and a couple of chairs into a "Smallish V ahape" and work inside this "Confined V Shape" to fold the sail. Also, I would have four ropes tied to my self, front, back, & off each side, with four people each in turn pulling me each way, to simulate folding the sail on the pitching bow of the sail boat.
"THE BAG HAS TO BE BIG ENOUGH" And; we never fold the sail on the fore deck as we will on the living room floor.
However I like "Sausage Bags".