Until recently, it was thought that should the size of Jupiter exceed 86,881.4 miles in diameter or the mass 318 times greater than Earth's, it would become a dwarf star, not a planet.
WASP-17b is twice as wide as Jupiter but only half as dense, and that's likely because of its gravitational proximity to its star, which is close enough to be categorized as a "Hot Jupiter".
A really mind-boggling discovery is HD 106906b. Its mass exceeds Jupiter's by eleven times. Which raises the question: How can we be sure that HD 106906b is a planet as opposed to a wandering dwarf star?