As advised by reviewers, I bought a large because I'm normally a size 10. When it arrived I wore a thick jumper then tried the jacket on. There was plenty a space there, even with my jumper on, but the arms were way too long ! I could tuck my hands in, that's how long they were. But you could argue it's because I'm short (5' 3"), which may the reason why the jacket appears mid-thigh length on the model, but on me it was a knee length jacket.
In a nutshell, it wasn't flattering to my figure, so I sent it back, and it was one of those days I wasn't in the mood and couldn't bother going through the trouble of ordering the size down to see if it would fit. So I'll never know if the arms and the overall length would fit better if I got a medium size.
However the jacket is lovely, in colour, fabric, and paddedness. It is great quality. Just not fit for short people (I think). My advice, if you're a size 10, is to get both the medium and the large, then send back the one that doesn't fit.
Amazon's customer care has been brilliant, as usual, and gave me a refund with no fuss.