Why could a gas planet not get much larger than Jupiter? What is the maximum size for a gas giant?
Usually they can get is Almost twice.  Here's the evidence:
 It is difficult for them to become even bigger than that as once they reach 15 Jupiter masses or so there is enough pressure in the core to ignite deuterium fusion, so those are considered “brown dwarfs” rather than planets.
An example is TrES-4, which is located 1,400 light years away in the constellation Hercules. The planet has been measured to be 1.4 times the size of Jupiter, but it only has 0.84 times Jupiter’s mass. With such a low density, it is dubbed the puffy planet.
It was discovered in 2006 orbiting a brightish star 1500 light years from Earth. Its diameter of about 1.8 times Jupiter’s makes it the largest accurately measured planet. Strangely, though, TrES-4 is very light for its size. It has only 88 per cent of Jupiter’s mass, giving it a density of roughly 0.2 grams per cubic centimetre, less than that of cork. Just how a planet can be as fluffy as TrES-4 remains a mystery.