Measure the size of your doorway and add half an inch. You will loose half an inch once the Stopper is filled.
I like to use Rice. Many people worry that this will attract mice. I have not had any problems with this but if you prefer you can use a kitty litter called "Feline Pine' This is Pine in pellet form. This works well as there is very little dust from it.
Whatever you decide to use, pick something that has some weight to it. The Stopper has to sit snug up against the door/window to block any drafts.
Do NOT fill them with sand. This can stain the fabric.
You should fill your Draft Stopper to about one inch from the top. The Stopper should NOT be filled to the point that it is rigid. Your Stopper should be flexible enough to fall slightly into the gap you are trying to fill.
I want to keep the shipping costs as low as I can for my customers. Draft Stoppers filled are very heavy and expensive to ship.
Also customers have different preferences as to what they like to use in their Draft Stoppers.