"...How big is Sedna? In our discovery images, we see only a point of light. We can't directly measure the size of Sedna from this point. The light that we see has travelled from the sun, been reflected off the surface of Sedna, and come back to us where we can see it in the images like the discovery images below. So a small icy object and a large coal-covered object, for example, would both look about the same brightness in the discovery images, because both objects could reflect about the same amount of sunlight.
We can measure Sedna's size using a thermal telescope, which measures the heat coming from the surface. We know how far away Sedna is, so we know that the surface temperature is about 400 degrees below zero Farenheit. A large object of that temperature will give off much more heat than a small object of that temperature (just light a match and a bonfire are the same temperature, but a bonfire keeps you much warmer at night because it is so much bigger). In collaboration with Frank Bertoldi at the MPIfR Bonn, we used the 30 meter diameter IRAM telscope, and in collaboration with John Stansberry at the University of Arizona and Bill Reach at the Spitzer Science Certer, we used the Spitzer Space Telescope. Sedna was too small to be detected in either. This tells us that Sedna is at most about 1800 km in diameter: about halfway in size between Pluto and the largest known Kuiper belt object Quaoar. Even though all we know for certain is that Sedna is smaller than 1800 km, we have evidence which suggests that the size might be pretty close to this number. We are virtually certain that the size is larger than the 1250 km size of Quaoar, though this object has shown many unexpected characteristics, so we can't completely rule out a smaller size.
Sedna
http://www.nineplanets.org/sedna.html
"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."