What is the largest known planet in the universe?
TrES-4, about 1.7 times the size of Jupiter.
TrES-4 , nearly twice the size of Jupiter, is believed to have an average density comparable to balsa wood. It's a gas giant, of course, and it's about 1,400 light years away. It orbits the star GSC 02620-00648 at a ridiculously high speed.
I know what you're thinking: "Enough about all these freaky gas giants! What's the biggest planet I can walk on?" The largest rocky (Earth-type) planet discovered to date is BD+20594b, an estimated 2.23 times the radius of Earth and 16.3 times its mass.. That size makes it around the size of Neptune. It was previously assumed that rocky planets could get no larger than 1.6 Earth radii, and this one would be half again bigger than that theoretical upper limit.
Bear in mind that exoplanets are spotted by noticing a "wobble" in a digital image of a star, and they extrapolate an awful lot of detail from those little wobbles. The study and cataloguing of exoplanets has only been going on since 1988. There's also kind of a fine line between a gas giant planet and a brown dwarf star. The largest known planet is Jupiter and the largest known rocky planet is Earth. But some of these exoplanets being spotted by HARPS and the Kepler Space Telescope are looking pretty good!