I also consider the easiness of the screens part of the size. (my opinion) For instance, in the Explore challenge, some screens were easy to pass by, but most would took you quite some time to get past them. Though the level doesn't have much screens, probably around 40 or so, I would consider the level medium sized based on estimation, as the playtime of the level is affected. True, playing the level for a second time when you know all the secret passages, will make it very short. I know this is not what people expect, but this makes me think that perhaps the playtime of a level from beginning to end could be some kind of indicator.
Like: playtime from 0 to 20 minutes = small, 21 to 60 minutes = medium, 61 minutes and longer = large.