There are three things that guide me when I deal with this.
1. I want her to genuinely know size, not just hers, anyone's doesn't matter.
2. That there are loads of other things apart from body and size that she can and needs to spend time wondering about.
3. That she is healthy is the most important thing. After my initial confusion cleared, I decided to write her a story that will subtly talk about size without talking down to her. I have no idea what the story is going to be but it is what she loves more than anything else in the world, so maybe it will speak to her. Two friends suggested I show her achievers, just sort of slip it in, who are different in size so that she knows it doesn't need to hold her back, in case she ever comes to a point where she starts to believe her size needs to stop her. But the best advice came in the form of this: http://idiva.com/opinion-iparenting/dont-call-me-fatty/24222