You'll also need to think about the angle between the mast and boom, if there is a boom at all (you can rig the sail without one. Unlike sailboats, many trawlers set the boom cocked up at the end. Then you will need to tell the sailmaker how the sail will be attached to the mast and boom. Will it have have a free luff, attached only at the head and tack, or will it have slugs, slides or shackles? How about roller furling? Likewise, will it have a bolt rope along the bottom or will it be set loose-footed, attached only at tack and clew?
You'll need to firm up the idea behind the purpose of your steadying sail, which is generally cut flat and with a straight leech, though there certainly can be battened sails with a roach and full-bodied sails which draw, too. Is it mainly to dampen rolling? To reduce tacking at anchor? Or do you want some thrust from it?
As has been said, the loads imposed are enormous; is your rig strong enough to carry sail?
Anson & Donna
Be kind whenever possible. It is always possible. ~The Dalai Lama