They would get much larger given the speeds of the particles that they contain. For some systems such as planets orbiting stars, or stars orbiting each other, the objects can become unbound and the parts can actually fly away. The Earth orbits the Sun at a speed that is just under the escape velocity from the Sun at this distance. If the force of gravity was halved, its speed would be exactly the escape speed. In fact, any body orbiting in a circular orbit would become unbound if the force of gravity was reduced by a factor of two!