Honestly, I doubt any universal definition will ever be satisfactory to everyone involved. People interested in celestial mechanics will always insit that a moon orbits a planet, and a planet orbits a star. However, people more interested in the composition of the planet will look at Titan, and be hard pressed not to call it a planet.

Edit: As to the size limit for a moon, I propose that we set the minimum at a size where a human could walk/jump on the surface without risk of flying off into space, and without noticably altering the orbit of the "moon". I am not sure whether all of the currently identified moons would meet this size limit.