The largest sold to consumers through normal channels is a gallon. Typically used by people who are feeding a lot of people, like making a dish for a large family gathering, or by people who only go shopping once a month or less. Some people live an hour from the store, so they just buy an excessive amount of food and shop infrequently.
Every barrel you see on TV or in a movie is 55 gallon or 42 gallon.
Other sizes exist but those two are so prevalent that you really only see those. The size comes from the oil industry where it's a standard unit. It's common to sell oil in different units, but the barrel size is so common that everyone just uses the same container and maybe just rounds the units.
Can't use a 42 gallon barrel , has to be the metric 160L, or the 200L 55 gallon drum.
Nah, the ounce is a unit of weight and volume. 55 gallons = 7040 fl oz ~= 7040 oz ~= 440 lbs = 200kg.
Aren't American units great? Here's a nice converter for various things, including mayo, which is ~0.94 oz per fl oz. But generally speaking, a fluid ounce is roughly the same as a weight ounce, kind of like how a milliliter is roughly the same as a gram (exactly equal for water, while a fluid ounce isn't exactly equal to a weight ounce for water).
I prefer the 300 gallon IBC tote. Then I can unload it from my pickup truck with the pallet forks on my skid-steer and put it straight into the mayo door on the side of my house.