I usually wear an XS or S and the small fits perfectly!
I have a small bust so it's great that the arm holes aren't huge. The fabric and lace is soft and stretchy, the straps glide smoothly and are easy to adjust and best of all the tanks are very long. At least they are for me, I am 5'3". I do not want any risk of my back being exposed if I have to bend over, so these are amazing! I'm buying more now.