Cessna is a manufacturer that has manufactured many, many different aircraft models. One model I am familiar with is the Cessna 172R, which can hold 56 US Gallons, 53 of which are usable.
Assuming you mean the cargo and not the fuel to power the tanker... 172 million gallons of oil. (42 gallons per barrel, times 4.1m bbl per the article below)
"Supertankers" are generally defined...