I think the kid is just over exaggerating and giving the horse a huge release.
I also think that at 4 years, she should just be introduced to jumps/jumping.
Depending on your build and the horse's build will tell you what size you can get. I'm 5'10 and have looked fine a very stocky 15h mare, yet looked very awkward on a 16.3 h very narrow gelding. Not to mention it depends on how you are built as well.