It's a bit tricky to say definitively what the "farthest planet" is since we're still discovering new objects in the outer solar system. However, as of now, Eris is considered the largest known dwarf planet beyond Pluto.

Here's why:

* Eris is larger than Pluto: Eris has a diameter of about 2,326 kilometers, making it slightly larger than Pluto.

* Eris is farther away than Pluto: Eris orbits the Sun at an average distance of 68 astronomical units (AU), compared to Pluto's average distance of 39.5 AU.

While there might be other dwarf planets farther away, Eris is the biggest and best-characterized one we know of beyond Pluto.