Complex sentence: In the U.S. the size is designated by their nominal capacity of the gas they contain when expanded to 1 atmosphere, 80,100,120 cubic feet, etc., with the most common being the "Aluminum 80".
Split into simpler sentences: In the U.S. the size is designated by their nominal capacity, the volume of the gas they contain when expanded to normal atmospheric pressure. Common sizes include 80,100,120 cubic feet, etc., with the most common being the "Aluminum 80".