I was taught to size the pillow case the same size or slightly smaller so that you get a nice plump pillow. It depends on what kind of insert you have for the pillow. Is it a knife edge pillow form or a foam square with a box shape? There was a neat link on QB this week for a pillow that used a men's button shirt front for the pillow back. The button placket was centered into the pillow back for putting the pillow form in.