[math-fun] the volume of an inflated square of rectangular bag.

Hello everybody, I always been puzzled by this fact : I just went to the grocery store to buy plastic garbage bags. They sell it in various sizes in liters but the format is in inches like 26 x 36 inches. That model if I recall has a volume of 121 liters. But actually if I have a square very thin plastic bag and I inflate the bag with air what is the shape of an inflated bag? (square of rectangular). Is this problem has been addresed before? What is the volume? of an inflated 1 meter x 1 meter bag. for sure the surface is 2 square meters. Any idea someone? Simon Plouffe