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.