Mine's a UK size 4, which is equivalent to a US size 0. I could have fitted into a UK size 2 but want to have the option of wearing a bulky sweater or plenty of layers underneath. The size I bought does still look neat on me and not oversized at all. The shoulders are, very importantly, a good fit.
Hope that helps. I can't wait to find out what you end up choosing. Do let us know!
ETA: Just in case the fit is different (although I don't think that it will be), my trench is the Heritage Gaberdine version in honey with leather epaulettes and cuff straps. I love it and it gets loads of compliments whenever I wear it!