What is the largest man-made object to be lost in space ? ?

The largest would still be the massive S-IVB stage, which is still out there in solar orbit again, practically lost because we still can't tell when it returns and where it is now.

The next biggest would be the SOHO sun probe, a bit over ten year ago. After a massive attitude control failure, caused by wrong ground commands, and the resulting loss of power and telemetry, it got lost and required a concentrated search mission by all large radio telescopes available to find it again. Finally, it was using the 300m dish in Arecibo as huge radar which allowed finding it again and measure it's orientation and rotation rates. Later the DSN was able to get a weak signal from SOHO every time the solar arrays had been producing a bit of energy for the systems to operate. During these short phases, it was possible to send commands to SOHO and make it recharge it's batteries and finally communicate with Earth.

The reactivation of SOHO is still considered a master piece of mission operations, because it was pretty much the return from iceberg to spacecraft. SOHO is still out there, watching the sun for us.

The largest objects we probably couldn't find in a hurry would be some of the Pioneer probes IMHO

Lost by virtue of how far away they are by now!

in 1997, due to the hopeless overpopulation of the Earth, the Robinson family and a pilot, take their spaceship, the Jupiter 2 to Alpha Centauri, but end up...

wait for it...

"Lost in Space"

a 1965 Irwin Allen production starring Guy Lombardo and June Cleaver. With Will Wheaton as the Robot.

I don't know about 'accidentally', but Skylab was a fairly large object that re-entered due to the inability/unwillingness of the U.S. to build and/or fund a launch vehicle.

Probably Challenger or Columbia.

Although technically they weren't lost.

They just had to look really hard to find all the itty bitty pieces.

At $10,000 a pound, one does not generally lose stuff. But spent upper stages of Apollo rockets have gone into solar orbit and people have lost track of them. The upper stage of Apollo 12 re-entered the Earth Moon system, made a few orbits, and was ejected. At first it was thought to be an asteroid. The upper stage is really shiny and bright, so people thought it was bigger than it really is.

Does faeces count as man made object. I am sure astronaughts have to relieve themselves and where does it all go? I am sure they dont save it till they come back to earth.

I don't know, but I can't wait to find out.

This is the biggest incident I can find - a proton rocket failed creating vast amounts of debris. Since it exploded, I'm not sure it fits your criteria exactly.

http://www.spaceflightnow.com/proton/arabsat4a/

This one broke up when re-entering the earth's orbit:

http://www.space.com/missionlaunches/080225-top10-...

The Tardis,

"it is far bigger on the inside Dr"

I think it was some video camera stand. Wasn't it? lol