In her gifts for knitters The Yarn Harlot talks about some beautiful project bags, but I also know without a doubt that she uses millions of zipper bags too! In fact, I believe that it is a well known fact that knitters use many more plastic zipper bags than any other type of person in the world.
I use the gallon size to store my stash in.....yarn in zipper bag, zipper bag in plastic tub with little cedar chips in the bottom to ward of, the dreaded killer of wool, moths! Quart size can sometimes hold leftovers and they make some bigger ones that you can put an entire sweaters worth of yarn in.
Think more when buying for your knitter, one box would not be enough!!