I think that the subjects size relation to the rest of the image works well - it just means that you'll have to print it at 20x30. And I love the depth of field. I agree that you may want to bump up the light and perhaps saturation. You may also wish to consider a crop that removes the sky above the trees.
I agree on the comments about a tighter shot - having the girl between the dark green and the blue without the sky would be fantastic. The colors are terrific and her playfulness really makes the shot.
I think its wonderful. The idea seems to be a child in a vast field of flowers. I can't see how a tighter crop would help that. In fact it would ruin it. In this shot, the OOf forground>in focus child>OOF background leads one into the girl while painting a softly moving perspective front to back. Nice. I wouldn't crop it at all, she's positioned wonderfully within the comp.
Only critique I'd make would be to start posting these bigger here, 600x800 for verticals is a good size, 800x600 for horizontals. You can go up to 900 pixels in width without having the site SW resize down.