Officially known as 2003 VB12, this object is the most distant body known that orbits our Sun. It is at present over 90 AUs away, 3 times as far as Pluto.
Sedna is about 1800 km in diameter, slightly smaller than Pluto.
Sedna is definitely not the "Planet X" that many astronomers anticipated before the discovery of Pluto. Planet X was supposed to be a much larger object.
Sedna is not even officially a planet at all. That classification decision is up to the IAU and they are not likely to decide to do so.