My G3si cabinet is:
Wide 2ft 3in
Tall 1ft 18in
Long 1ft 22in
I've heard people say to install them at least 4 ft away from the wall, this would take your length to almost 6ft, however mine is only 1 1/2 feet from the wall - -it does not allow for any access to behind the PBX to attach or manage cables/wiring - - extra 4 ft sounds good to me)
I have 3 cabinets all stacked on top of one another secured by the earthquake bars.