What is a K-bottle?

There is a high pressure gas steel cylinder size K. Other MFG designations are 300, 49 and 1L.

The K-bottle has a diameter of 9.25 inches (235 mm) and a height of 60 inches (including valve and cap), 55 inches (1.397 m) without valve. The internal volume at 1 atm is 1.76 cubic feet (49 l). The U.S. DOT specification is 3AA2400. The service pressure is 2400 psi (165.4 bar). The weight is 139 lbs (63 kg).

Numbers from Wikipedia.

The k-bottles shown in the Shuttle drawing have the same typical ratio of diameter to height.

Table from Advanced Specialty Gases

They used heavy ordinary standard industrial gas cylinders, no sophisticated lightweight carbon fiber overwrapped spherical gas containers. The Enterprise shuttle was used for gliding and landing tests only, so no rocket science gas containers.