Well I am short and a bit on the round side, so I ordered a 14 and a 16. (I am about a 14 normally)
The 14 fitted perfectly, but I kept the 16 in the end as I wanted to be able to wear it with a thick hooded sweatshirt underneath, think Cornwall summer weather 
So you might want to order a couple of sizes too, just to be sure you get what you want. I absolutely love it 