I have a 2008 Torrent (Pontiac version of the Equinox) You didn't mention the year of your vehicle so the space may be different for the newest style of Equinox. In mine I have a plastic crate bungeed behind the drivers seat. My guy is 80lbs and he travels in a plastic crate that is approx. 28" high x 24"wide and 36" deep.
Keep in mind when you are travelling they don't need as much room as they do for their house crate.