Choosing your sizing is always the hardest factor with rash guards. I typically wear a woman's size 10, so I choose a large because I didn't want a tight fit.  This turned out to be a bit loose, but it's super comfortable and as I am diving and snorkeling a lot it's perfect for me.  I would be curious to see if a medium would have still been comfortable.  I really like the flexibility the front zipper gives me too.