I ordered an Ultraviolet, large size. I'm 5'7'' and 135 lbs. I'm usually in between medium and large for leotard. I think medium will suit me as well. This piece does not breath well due to the fabric, so I usually wear this in the winter with a warmup sweater.
I bought this leotard in teal for the summer intensive I am doing this summer. I loved the open back so I could make sure to engage my back doing port de bras and such. I am 14 years old, 5'4" and 97 lbs, and I normally order P/XS in leotards to have a high leg line to make my legs look longer; however, I am very picky because I do not like plunging necklines. When this leotard came and I tried it on, I instantly loved it! The color was beautiful and the leotard fit perfectly. The leg line came up very high (but not showing my derrière) and the neckline covered very elegantly. There was plenty of coverage in the front for me (I wear 32A under my normal clothes). I could not wait to tell all the girls in my studio about the leotard! I liked this leotard so much that I am buying another one in black.
I hope this helped! <3