Size up, unless you want it to be very fitted!
I usually wear a size small in tops, size 6 in jeans, and medium in dresses. I ordered a large after reading reviews, and glad I did! It is a beautiful color, flattering, and isn't too short. Since I sized up, it fits great! This dress is versatile and could be used as a formal dress with a nice set of heals, or dressed down with the addition of a jean jacket and some wedges on my feet.