I don't have this but do have a few of her other designs and always find that they are generously sized. It is described as semi fitted so the finished measurement at the bust should be 10 1/2 - 12 1/2 cm (4-5 inch) over and above your measurement. Personally I would go for medium and increase at bust if need be as the rest of the garment seems pretty loose.
Amended, sorry forgot to say that a lot of the reviews say that the sleeves are tight.