What is the biggest of space junk in orbit? What was it used for and is there a picture?
The UCS has a list of satellites which can be sorted by launch mass.
The top 10 is mostly spy satellites for which it's difficult to determine if they're active or defunct.
The heaviest satellite of which I'm sure it's not functional, is Envisat at 8 tons.
(1:1 model of Envisat at Space Expo in Noordwijk, the Netherlands)
This list doesn't account for upper stages etc., but even a big upper stage like the Ariane 5 ESC-A weighs only 3 tons empty. The Saturn 5 upper stages are bigger, but aren't in Earth orbit.