I bought the small for my (then) 10 lb toy australian shepherd puppy and he didn't take to it. The size and shape were not conducive to his little mouth and it was a little too heavy and thick for him.
I ended up giving it to my German Shepherd who LOVES balls and can be gentle with her toys (until she isn't ...then watch out!). She is gentle with this and likes to just play catch.
I would recommend this for a medium size dog and not for aggressive chewers.