I am 5'6" and 115 lbs.  I wanted this top to be oversized and long enough to wear with leggings.  I ordered a size Medium.  I am happy with the fit and material.  It covers my bottom completely.  Just a note: this is not 'sweatshirt' material.  It is a bit thicker than t-shirt material and soft.  I washed it in cold water, gentle cycle and dried on low.  (The care tag recommends dry cleaning.)  It shrunk very little (if at all) when I dried it.  I am happy with this top for the price and will probably be ordering more.