I've been eyeing a soft draped trench from Club Monaco for a while but I've never bought anything from this store and need some sizing advice.

I'm small on top and I prefer a slim fit on my shoulders so I can't decide if I should go for the XS or S or M. What do you think?

I'm a pear and I generally fit a 2-4 on top and 6 on bottom. Most recently I bought the utility jacket at BR in XS and the white blazer in a 0, but it runs exceptionally big.

TIA for your input!