The largest planet ever discovered has been found orbiting a star 1400 light years from Earth, but also has some additional unusual features along with its size. It is 1.7 times Jupiter's diameter, but has an extremely low density of .2 gram per cubic centimeter. A handful of similar "puffy" planets have been found, which has astronomers scratching their collective heads - this class of planet is not easily explained by current planet formation theories. The planet's has a quick orbital period of 3 1/2 days and was discovered by the transit technique.
ScienceDaily has some more details here.