Here is the website for the Lookout Seats and you should be able to get measurements and ideas from the site. I bought mine on ebay.
http://www.snoozer-dog-beds.com/dogcarseats.htm
My larger dogs prefer to have their own seat so they can either curl up and go to sleep or just hang out in it. The little ones are okay with 2 in them but I get more "she touched me, no he touched me" comments from the peanut gallery when we go on road trips.
I have 3 Lookout seats to cover the entire backseat so the gang can all travel safely if we have to.