Pluto may be the biggest Kuiper Belt object again. Sky and Telescope has a news item by Kelly Beatty describing observations of an occultation of a 17th-mag star (named L1654635357407160223) by Eris set new and smaller upper bounds on Eris' size.
…successful observations from widely separated sites create two chords across Eris's shadow that yields a unique solution for its diameter (assuming that the object is spherical).Previous observations were based on direct infrared measurements and size estimates were derived from assumptions about the surface brightness (albedo) and other aspects of the body.
That number, according to Bruno Sicardy (Paris Observatory), is hard to pin down exactly because timings derived from the three telescopes' light curves have some uncertainty. Even so, Sicardy notes in an email, "Almost certainly Eris has a radius smaller than 1,170 km" — and that would make it ever-so-slightly smaller than Pluto, whose diameter is thought to be 1,172 (±10) km. Don't be surprised if the final value gets pushed another 50 or 60 km lower.
Image credit: Marselo Assafin and others. From Sky and Telescope.