Jupiter, the largest planet in the solar system, is a popular size. Many heavier planets in other star systems are about the same size, as are most brown dwarfs (objects that are more massive than planets but not massive enough to shine as true stars) and the lowest-mass stars, known as red dwarfs. The size is dictated by the object's mass and processes in its interior. As an object gets heavier, gravity squeezes it more tightly, making it hotter; radiation pressure pushes outward, keeping the size tightly controlled. The Sun and Earth are shown for comparison. [Planetkid32/Wikipedia]
Find out more in our radio program entitled Moon and Jupiter.