Is there a lower limit to the size of a gas giant planet?
Yes. But it depends on its distance from a star (solar winds will strip away the “lighter” elements and send them outward). There is a reason why a planet as small and close to the Sun as Mercury does not have much of an atmosphere. But massive planets farther away do. If the object were far enough away from any solar or stellar winds, the lower limit would be whatever minimum mass were required for it to be roughly sperical in shape, and be able to bound itself together with its own gravity. The upper limit is a gray area, usually around 12-13 Jupiter masses gets you in the Brown Dwarf star category, 80-100 Jupiter masses will get you to a Red Dwarf, which are the lowest mass stars that go through main sequence Hydrogen Fusion.
