Wow... 5'4"? I always say go with what your fitness goals are. If you see yourself in the mirror and plan on bulking up a bit, get the medium. If you would rather slim a little, get the small. I wish someone told me this back when I got my raw denim jeans so tight before I ever bulked up.
Thanks for the sizing advice.
Concerning the website, it only offers a restricted choice in terms of size