Mekong River, Laos
The boats of the Mekong (at least on the stretch between Huay Xai and Luang Prabang) come in many flavours and colours, but primarily in four different sizes. First there are the huge rice barges, which dwarf anything on the river. Then come the smaller “slow boats” that ply tourist routes; they hold perhaps 80 people in a sardine-like array.
These aren’t the smallest (those are long tail boats used by fishermen) but they seem to be the staple for local trips. They can hold a dozen or so people, and are responsible for the short haul tourist routes.
Say, between Luang Prabang and Ban Xang Hai or the Pak Ou caves.