Agreed that we would need more info and pics would be great, but what HD describes (3 ft X 3 ft) would hold close to 160 gallons. A standard 55 gallon drum is 22.5 inches by 33.5 inches. I'm not even going to get into the fact that cauldrons have a rounded bottom and how that affects the volume.
Total guess here, but the "15" may be the volume in gallons. We need pics or at least some dimensions.