I'm 5'1" and weight 115 lbs.  I always wear a size small but after reading some of the reviews I thought I should buy a medium.  I just got it and tried it on.  It is so small (marked M) that I couldn't even button it.  A large probably would fit but I'm not impressed with the material or obviously the sizing so I won't be buying another one.  This one is now in the goodwill box.