Welcome! No bus yet? You've come to the right place. This is a great source of information not only for conversion how-tos but help to identify the best bus type for your needs.
I looked at a 1971 Carpenter and the outer rear door width was 2 ft 11 in. (35"). I didn't measure the actual opening width, but I'd estimate it to be 33" (one inch less than exterior seam on each side). I have a chart showing dimensions of a 1959 Superior body and it also shows a width of 33". School buses are built to standards so this should be universal (measure before buying just in case).
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I've measured mine (but don't remember) and I do know that our 12 cf upright freezer will definitely fit thru (how do you think we got it out to NM???)... as will a standard washing machine and dryer. We put the 30" range thru that door already as well. The PITA factor is laying the appliances down enough to fit thru the door height wise and still keep it moving (rough on a bad back). Thank heavens we kept our hand truck. That and those furniture mover sliders were used extensively to load the bus with all our junk to come out to NM.The worst was the TV (60" wide and it fit thru the emergency door with about 1/2" to spare). That won't be going back in!