it looks like she's wearing a small...
i'm 5'9 and found small to look really small... i got a medium which is perfect!
I've ordered in the medium on it's own but it's taking ages- didn't see the need for another 16inch chain and apparentely they don't get the medium open heart in to sell on it's own VERY often. I do like my small one but theink i'll actually prefer to wear the medium more often- I seem to favour wearing my RTT Heart tag on a ball chain much more often than the open heart. Probably because it stands out more.
Actually I own the (new) mini, the small and the medium... I wore the medium on my wedding day! And I'm POSITIVE she's wearing the medium. The small is still very small - I'm sure you wouldn't be able to make out the shape of it from the distance those pictures were shot at! (am a big Bridget Jones fan as well and love the necklace!)
The fact that she is wearing a medium and it still looks small makes me want to get the medium now! (I am deciding between small and medium rose gold)