Personally, I like a size of about 180+ km for a true moon since that is about the radius above which most most moon's in the solar system have enough mass to take on a spherical shape. But that's just my preference. I can certainly see why some would be upset to have the 100 km radius objects not be considered moons. Why not save the "moon" designation for the natural spherical objects orbiting planets and call all the other objects "satellites".