I'm sorry, but that is a pretty unreasonble question. You can get pretty inexpensive blankets from Dover, Smart Pak, Bit of Britian, or Universal Horse. Each horse should have it's own blanket so that it does not rub. If the blanket is to small or too large it will rub and hurt your horse.
I don't think I will ever run into the problem having one of my horses rub - because they would never wear the blanket long enough. However - I would like to be able to keep it on the horse, though :)
I think I will go with a size 78 - sounds to me that this could work after reading all your helpful replies - thanks again!
One more question - canvas or denier?