I have that dress. I am a consistent 4 in dresses. I got a medium in that dress because that was the only size I could find when I got it. It fits ok, but I wish I had gotten a small. I don't know if that's helpful at all--I guess it is like FP said--TTS to a hair small.
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Know first, who you are; and then adorn yourself accordingly.
-Epictetus