Here's the link to high res testing shots that someone asked for (Click zoom, then show all sizes, then click original for super high res)
http://www.flickr.com/photos/f1photos/
The red-bull is pretty long from cockpit back, hence why they have more room to package the rear before the end of the car, McLaren seem to have a longer car from cockpit forwards for some reason. Of course the super short (not necessarily skinny) packaging of the RB is the main contributor to the acres of floor space at the rear.