For starters: my size:  I am 5'10" 137.8 lbs, 36C chest.  In many brands I'm usually a small/medium or 4/6.

I absolutely love this brand, I have 12 of their shirts. High quality. Always.  I've found the sizing a little odd though. I have 3 of their tank tops. They are true to size, but there is a little shrinkage, not much though.  so I get them in a medium.
I have 6 different long sleeve shirts and 3 sweaters. They seem to run a little smaller, so I always get a large so they shrink to a medium after washing (cold water, low heat setting).

My height is probably a big factor on the form fitting clothes, so if you're under 5'8", you might not have the same problem I do.