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.

55 gallons is about the size of the average American, or something in the neighborhood of 200kg.

The explodey barrels in fps games are typically 55 gallon drums

I have absolutely no idea how big 55 gallons would be

Something a bit larger than 200l... I think an oil barrel is around that size (and yeah, it's "the standard unit" for that).

In all seriousness, if the GP said it was a 2 gallon container I would honestly not know if it was true.