Agreed, this is the pragmatical aspect of mapping small waterways. Most mappers do not want to waste time to measure width, depth and flowrate of a small water channel nor of a stream or river and that is why we have more generic tags for those.
Under this aspect those tags in use make sense to me:
drainage channel wide, lined = drain
drainage channel small, unlined = ditch
drainage channel very small, prefabricated = drainage_gutter (as proposed here).
natural waterway wide = river
natural waterway narrow = stream
artificial (fresh)waterway wide = canal
artificial (fresh)waterway small = ???
From that point of view we are just missing a tag for small freshwater channels. Still looking for a good idea how to tag those … 