I'm 5'2", 130 pounds, and I got a Medium as my back is my biggest measurement.  It was a tiny bit too long with flats and sandals and a little bit low cut (tragedy of the wide back small breast frame); however, as a casual dress, it's been great.  If I need to run around, I've knotted the bottom.  I've been wearing a tube top or a cute sports bra underneath and there are no modesty issues.  I took it travelling, and it just needs a shake to get out the vast majority of wrinkles.  The only issue is the side zipper.  It has snagged the fabric, and I was worried that it would tear.  I spoke to customer service, and they gave me a small credit so I could get it repaired by a seamstress.  It was much appreciated.