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In 1992, astronomers detected a faint speck of light from an object about 42 AU from the Sun — the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,300 KBOs have been identified since 1992. (They are sometimes called Edgeworth–Kuiper Belt objects, acknowledging another astronomer who also is credited with the idea, and they are sometimes called transneptunian objects or TNOs for short.)
So, NASA says that TNOs are Kuiper Belt Objects. And, by this Wikipedia article, it says that as of now, we think that Pluto is smaller than Eris. I'm not good at changing articles, o somebody needs to check this, and maybe put it in if it's okay. Thanks!
Looks like a close contest! Wikipedia say Pluto (dwarf planet) 1184±10 km, Eris (dwarf planet) 1163±6 km. But by mass Pluto (1.305±0.007)×10^22 kg, vs Eris (1.67±0.02)×10^22 kg, so Eris is more massive!
This recent article  also saying its too close to be sure. I'll also let someone else sort it out. Tom Ruen (talk) 05:06, 14 April 2015 (UTC)
The whole TNO/KBO debate is getting tired. Wikipedia uses the IAU for guidance on this, and the IAU divides TNOS into kuiper belt objects and scattered objects. Pluto is a kuiper belt object, Eris is a scattered object. Serendipodous 07:09, 14 April 2015 (UTC)