The smallest size of oxygen tank available is an M-2 tank, according to Applied Home Healthcare Equipment. An M-2 tank holds 42 liters of oxygen at 2,200 pounds per square inch of pressure.
As of 2015, the labels on oxygen tanks include an “M," which stands for medical, followed by a number. The number corresponds to the amount of oxygen measured in cubic feet that the tank holds, explains Inogen. Portable oxygen tanks were formerly labeled with the letters "A" through "E," with "A" being the smallest size.
An M-2 tank weighs 0.7 pounds when empty, stands 5.3 inches tall and is 2.5 inches in diameter, according to Applied Home Healthcare Equipment.