Er,
Just a lucky guess...
Seemingly it was discovered by astronomers using California's Mount Palomar Observatory, way back on November 14, 2003.
Sedna, or 2003 VB16, as it was originally designated, will have follow-up studies to measure its thermal radiation to determine how hot (or cool)it is, and therefore provide a better estimate of its size. However the planetoid is usually cold; the temperatures never rise above minus 240 degrees Celsius...
There is indirect evidence that Sedna may have a moon. (rotates slower than expected... )
A notable feature of Sedna is its reddish colour (and it`s very shiny), which doesn`t help the measurements either...
If you want to see it, it`s currently 13 billion kilometres away, in the constellation Cetus (Position J2000: RA: 3h15m10s Dec: +5d38m15s), hehe, but over the next 72 years it will become brighter and closer.
After which it will head out into the oort cloud again, reaching a whopping 130 billion kilometres from the Sun, taking 10500 years for one solar orbit,
(NOTE: Er, you do need a 200 incher to see it though)...
:-o