Just a bit deeper about 4-6 ft..maybe as much as 8..
If you do any engine swaps you will need the room for an engine hoist..and of course the stuff expands to fit the available space..<Grinn>
As far as the main garage door tha tcan be changed to a single 16' door if you like. Having a basic plan like this is good to show a contractor or builder.. A lot easier to build when one can see the picture..
OMT