ORDER ONE SIZE UP.
I am very small and petite: 110 lbs, 5'1". I usually wear a size 2 womens, or XS (or a size 3 juniors). I ordered a "small" in this item thinking it might be too big because a small in ladies is still usually too big for me (I usually have to order XS). Well, not this item. It was just right fit even though I ordered one size up from my norm. So I hope it doesn't shrink.
HERE'S MY CONCLUSION:
If you are very tiny like me (size 0-2 ladies), I would order a small.
If you are small (size 4-6 ladies), I would order a medium.
Just a suggestion from extrapolation from my own personal experience with my size only.