Traditionally there are two sets of names used to differentiate between oxygen cylinder sizes. The original set uses an alphabetical system, starting with A for the smallest size and E for the largest portable size. The new naming system begins with the letter “M,” for “medical,” followed by a number that signifies the amount of cubic feet of oxygen in that can be compressed into the cylinder. So the original B cylinder is now often referred to as an M-6 cylinder because it can hold 6 cubic feet of oxygen. Below, see the handy chart that matches oxygen cylinder sizes with their dimensions, capacity and accessories.