New Horizons has found that Pluto is larger than its greatest rival, Eris. Last year, other observations suggested that Pluto was around 1,471 miles across, which would have meant it was bigger than Eris, whose diameter is 1,445 miles. Link: Pluto Regains Its Title as the Largest Object in Its Neighborhood Now New Horizons has determined a diameter for Pluto of 1,473 miles. Link: How Big Is Pluto? New Horizons Settles Decades-Long Debate. The finding means that Pluto is the largest object in the Edgeworth-Kuiper belt, contrary to pronouncements made a decade ago when Eris was discovered.