@CakeInMyFace I bought that dress last week, and it is beautiful. But I struggled with sizing, and couldn’t get a size to fit. I am a standard size 10-12, short (5’1”), and a bigger bust than you. I am not particularly wide at the shoulders, but found the size Small quite narrow there and tight across the upper arms when I moved. I then exchanged for a Medium, and although this was perfect at the shoulders/upper arms, the bust was too wide and looked weird because the bust darts fell at the wrong place.
I can usually wear a Toast size Small or 10, but have found that this season their sizes are coming up a little smaller than usual. So, I would go with what @Spottyegg said, and order a couple of sizes to try.
I hope it fits you, it is so beautiful!