There are two main belts- the one between Mars and Jupiter which is relatively stable, and the Kuiper Belt near Neptune’s orbit which is the source of most comets (Pluto is the largest known Kuiper Belt object).
Pluto's diameter is 1,485 miles. Eris, discovered in 2003, has a diameter of 1,850 miles. Eris' period is 560 years. It has a highly eccentric orbit, taking it at times inside the orbit of Pluto, but for now it is the most distant object found in orbit around the sun.