Im 52, 124lbs, 33 bust, with rather muscular shoulders for a woman. I usually wear an XS in juniors/petite womens clothing. I purchased this shirt in a S and was very pleased with the fit. Its tight, especially in the forearms, and a little difficult to get on, but it needs to be this tight for swimming. I have full range of motion once the shirt is on, and it does not restrict my movements. It stays in place while swimming. Were I buying this shirt for exercises other than swimming, Id order a medium.

I feel like the UPF of this shirt is effective. I usually wait until the UV index drops to 3 or 2 before I go swimming, but I had a light tan before I bought this shirt, and my tan lines went away once Id been using it for a few weeks.