Why not just buy for the big and then if you change your mind, you have what you need ..... or of course, it just becomes inventory for your stash!
Actually, it has been quite a while since I made the big one, and I can't even remember exactly what the yardage was for that. I am only an hour from home, but I hate to drive back just to get the pattern. I will probably just guess at it and buy generously, and as you say, have some for my stash !