well, a problem that I had when I was a barista (at a different coffee chain, back in 2001), was that we didn't have three sizes, we had four. So if somebody asked for a medium, I would get confused and say "well, we have two mediums", and if someone asked for a large I would have to ask "did you want the one that would reasonably be called large or did you want the one that is actually the largest that we have?". Whenever I tried to standardize "small, medium, large, extra-large" or "tiny, small, medium, large", then always half of the people would end up the wrong thing and get upset. It was lose-lose.
So now when I go to a cafe I say "I'd like a twelve ounce coffee", so I don't have to try to translate that through their local notion of whether that's "small" or "medium" or "tall"