There are three types of planets in the solar system so lets consider the largest of each type. This only refers to this Solar System and not to any Extrasolar Planets around other stars in the Universe.
Dwarf planets are a controversial group of planets. The grouping was created in 2006 just after the Eris had been discovered. The definition for a planet had not been standardised. The International Astronomical Union (I.A.U.) had gathered at their annual meeting to discuss what makes a planet. They came up with a criteria which had to satisfy all three, otherwise it would be known as a dwarf planet.
Eris was believed to be bigger than Pluto and some scientists didn't like the idea of another planet in the Solar System so they pushed for above criteria to be accepted. The resolution was passed and Pluto was downgraded to Dwarf Planet status. It is therefore Pluto is the largest Dwarf Planet in the Solar System.
There are four rocky planets in the Solar System and they are four nearest planets to the Sun. Mercury, Venus, Earth and Mars . The largest of these is our home planet, Earth. The equatorial radius of the Earth is 6378.137km, slightly bigger than the next largest, Venus. Venus for comparison has an equatorial radius of 6051.8km. Venus is often referred to as the evil twin of Earth, about the same size and age but with a totally hostile atmosphere.
The largest planets in the solar system are the four gas giants of Jupiter, Saturn, Uranus and Neptune. These gas giants are many times greater than the Earth. The largest gas giant is Jupiter and its Equatorial Radius is 71,492 Km compared to 6,378 of Earth. The radius of Jupiter is just over 11 times that of the Earths.
Gas giants have been referred to as failed stars, they don't have the necessary mass in order to start nuclear fusion. The smallest star ever discovered is smaller than Jupiter and is EBLM J0555-57Ab in the constellation of Pictor .
The largest overall winner is Jupiter which is the largest planet of any type in the Solar System. Even though it is the largest in the Solar System, there are exoplanets out there that are many times its size.
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