NASA's Hubble Space Telescope has measured the size of "the tenth planet" nicknamed Xena and according to scientists it's only a bit larger than Pluto.
Though previous ground-based observations suggested that Xena was about 30 percent greater in diameter than Pluto, Hubble observations taken on Dec. 9 and 10, 2005, yield a diameter of 1,490 miles (with an uncertainty of 60 miles) for Xena. Pluto's diameter, as measured by Hubble, is 1,422 miles.
"Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Xena," said Mike Brown, planetary scientist at the California Institute of Technology in Pasadena, Calif. Brown's research team discovered Xena, and their results have been accepted for publication in the Astrophysical Journal.
More details over at Space Flight Now. There are lots of large objects in the Kuiper Belt. From 'largest' to small: Xena, Pluto, 2005 FY9, 2003 EL61, Sedna and Quaoar.