How about judging a levels size from the play area as the player perceives it? (if that makes any sense at all)
That means two copies of a screen still only count as one, as long as the player is intended to think its the same place.
Take colors as an example. There are multiple versions of every area in the level, but the player is intended to think the whole thing is just one single island.
*cough* Gaia.
I never said LitS is bigger than Gaia. I said it is bigger than Natural and colors, which you listed as two of the biggest levels ever made.