I like that you've decided not to make either the girl, robot, balloons, or aircraft hostile towards each other.
I liked the style of the background and foreground, but I think in trying to make the giant appear even more... giant, you've cut out too much of both and there is far too much sky doing nothing at all. It's good use of a clever scaling tool, but too much of it.
You've done a good job with the clouds, though. The aircraft-stream gives a horizontal reference point so you can tell the clouds are behind the giant, on one level. The clouds that just touch the tip of the green hill throw this effect off a little: clouds that far away should be even fainter, smaller, and sparser.
Brilliant piece of artwork: the room left for improvement can only be a good thing, right?