I don't think there really is a formula. It seems to me it has a lot to do with the shape of the hull
and the sail area of the hull
. How and where you mount it may also effect how big it needs to be. How you attach your rode
may also effect the sailing at anchor
ability as well.
This would be a good topic for a bunch of 37 Tayana owners. Some boats that sail quite well also love to sail around the anchor others don't seem to require a riding sail. It varies my model as much as by make. Our prior CSY 33
never sailed at anchor.
If you have some scrap sail cloth you could try a few shapes base on how you can attach it. I've seen them between the topping lift
and the aft end of the boom as well as various ways of flying them farther aft using the main halyard
and guy lines holding the sail along the centerline. I'm not sure it has to be all that precise. I assume the smallest size that works would be easiest to handle and not be in the way at anchor.