Hi, Donna! This is a tough one! I have a large bust and broad shoulders. I normally take a size 8 (depending of course on the particular item.) I found this tunic to be a bit oversized. I tried both the 8 and the 6. The real difference I found was the volume in the body - not so much at the shoulders. I choose to go down to the 6, since I felt it was already voluminous enough. For me, I felt the larger size looked like it tented out (ever so slightly) at the bottom, whereas the size down was still tunic-like and flowy, but just a tad less volume.
I also loved the cream color, which only came in petite. For that one, I ordered the 8P (figuring that I would have to size up in P anyway.) They were sold out of the 6P, so I couldn't compare. Even the 8P is way plenty tunic-y and volume-y.
My feeling is the difference is not enough to make it feel like either would be the wrong size - I just preferred the ever-so-slighly less voluminous size down. I guess it just depends on how much volume you like!
(Whew! How's that for an obsessively detailed answer?)