You're welcome. I actually had a large open heart (the 1" diameter heart on 18" chain), and in my opinion, wished I had gotten the medium or small. My facial features are small, so the big heart in contrast didn't look as good on me. I think you should go with the small or medium just to be safe.
A suggestion, if I may...
Since everyone and their mother has this necklace, I would suggest jazzing it up a bit like this: get the medium version and add another small heart made out of a precious stone. I saw this on a girl yesterday and liked the effect.
Tiffany sells the open heart pendant in a variety of stones (amethyst, jade, etc) separately (i.e. no chain included).
Or you can just get her one of the Open Heart pendants and buy a Tiffany chain to go with it.