What is the largest known planet in the universe?

TrES-4, a gas giant about 1,400 light years away and nearly twice as large as Jupiter, is thought to have an average density similar to balsa wood. It orbits the star GSC 02620-00648 at an absurdly fast speed.

"Enough about all these freaky gas giants! What's the biggest planet I can walk on?" Well, BD+20594b is the largest rocky (Earth-type) planet discovered to date. Its estimated radius is 2.23 times that of Earth and its mass is 16.3 times that of Earth, making it roughly the size of Neptune. Previously, it was thought that the largest size a rocky planet could have was 1.6 Earth radii, so this one would be half again larger than that theoretical maximum.

It's important to keep in mind that exoplanets are discovered by spotting a "wobble" in a digital image of a star, and that they extrapolate an awful lot of detail from those little wobbles. It's also important to keep in mind that the largest known rocky planet is Earth, and the largest known gas giant planet is Jupiter. The study and cataloguing of exoplanets has only been ongoing since 1988.

With a radius of roughly 1.7 times that of Jupiter, the gas giant planet TrES-4b is currently the largest planet known to exist in the universe.