Nice discussion of Sedna, which is 3 times as far from the sun as Neptune and ~42-45% the diameter of Pluto. It is outside of the Kuiper belt and in the inner part of the Oort cloud. They think it’s round though. Discussion of what a planet is and how it could be defined follows. http://web.gps.caltech.edu/~mbrown/sedna/index.html
Possible ways of defining planet: historic, “Historic-plus”, gravitational rounding (the author prefers something like planetoid or sphereoid), and population classification. That last one is tied to the “Cleared its neighborhood” criteria and is currently accepted scientific usage. “Historic-plus” is the conventional definition: planets are defined as bodies that are equal to or bigger than Pluto.