I'm a 34D/DD, a 6/8 on top and a 4/6 on the bottom, but I'm just flat out curvy. I wear an M/L in the stretch jerseyish stuff and an L in anything that is more strutured.
If you're a small C, I think you should be fine with a S most of the time. I think you may only encounter problems when the dress is structured or a button down shirtdress, at which point you should probably size up and get the dress tailored.
I think we have very similar body types. I am somewhere between a 34 B and C and find F21 non-stretch dresses to be cut rather generously in the chest - I wear a small in them and normally wear an xs/s and a 1 or a 2 in most other stores.
I usually size up one for anything knit/stretchy or anything synthetic , but it really is all over the place. I can usually stay with a S for 100% cotton. I am usually a 1 at Gap, 0 at Express and BR.
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