Back in 2000, the planning and testing for the Rosetta mission was in full swing. First tests where done with full size models, in the largest vacuum chamber in Europe. Later Rosetta itself was rigorously tested under vibration, in vacuum, cooled down and heated up, to test if all system would function properly in outer space. ESA published a nice set photos from that era on which you can see, how large Rosetta actually is.
Rosetta on Earth photo series. Mouse over the photos to see the popup with additional information.
In 2002 ESA published a extended Rosetta bulletin with a lot of mission background. Download the Rosetta bulletin in PDF