Size is totally subjective. A level may have a huge number of screens, but have many of them be duplicates of other screens. Or a level might have a very few screens, but have so much in them that the level takes an hour to play through. There really aren't any official standards for grading a level's size- just tag your levels with the size you feel appropriate.
(Incidentally, I find it really, really irritating when people don't tag their levels properly. I have 121 pages of levels, and if I go to play a level, and then find out it's not tagged, and that the name it's filed under in the editor is different than the name in the .knytt.bin, then I have to dig for it. Which is not fun. Or go into the editor to play it. Which can be spoilery. The tags are there for a reason.)