It sounds like a quistion from QI And if you guessed the sun (right on the picture) then your wrong Please Register or Log in to view the hidden image!
Apart from the sun? Jupiter. The Kuiper belt isn't really an object in and of itself; it's more of a collection of objects. I'm guessing the "correct" answer is the heliopause?
Right now comet Holmes has an extended coma that exceeds the diameter of the sun. Visually it would be the biggest for about a week or so.