Yes, it only works for objects that you can estimate the size of. The tree is pretty easy (trees don't have that much variation in width, especially given that

Yes, it only works for objects that you can estimate the size of. The tree is pretty easy (trees don't have that much variation in width, especially given that you can determine the type of tree from other clues). A far away city is a bit more difficult but you can still get a rough estimate by guessing, say, how big some building is (or better yet how wide the road you're walking down towards the city is!).