When Eris was discovered in 2005, Hubble Space Telescope images suggested its diameter was about 1,500 miles, about 5 percent bigger than that of Pluto.
Astronomers got a chance last November to refine the measurements when Eris passed directly in front of a distant star, NewScientist.com reported.
Bruno Sicardy of the Paris Observatory and colleagues took measurements from two different sites in Chile, and found Eris' diameter is 1,400 miles, almost exactly the same as the best measurements suggest is true of Pluto.
"It could be smaller, it could be larger; basically, it is a twin," Sicardy said Oct. 4 at a Division of Planetary Sciences meeting in Nantes, France.
Eris remains the heavyweight of the dwarf planets, however, with much more mass than Pluto, he said.