Trying to find out the size of a compressed-gas bottle can be difficult. There often are no size markings on the cylinders themselves because compressed gas is sold by the pound, not by volume. A small cylinder may contain more compressed gas than a larger cylinder if the gas in under more pressure. But most bottles do come in standard sizes established by the U.S. Department of Transportation (DOT,) which you can identity by measuring the bottles' dimensions.
Measure the height of the bottle, including the valve and cap.
Measure the diameter of the bottle. The diameter is the width of the bottle. If you have trouble measuring a bottle's width because of its rounded ends, you can instead measure the circumference of the bottle, then divide the circumference by pi (3.14) to find the diameter.
Find a table that identifies standard-size bottles by their dimensions. Each standard compressed-gas cylinder has its own DOT specification number. For example, if you have a cylinder with a diameter of 9.5 inches and a length of 60 inches, you can look on a size table to determine that this bottle meets the DOT specification "3AA2400." This particular bottle has an internal volume of 1.76 cubic feet.