Measure the vehicle in use
Include the mirrors when checking the entrance width, and any roof-mounted equipment when considering height. The clear door opening matters more than the overall size of the front wall.
Once inside, leave enough room to open the doors and get out comfortably. Allow for boot or tailgate movement and any items you will regularly carry past the parked vehicle.
If you expect to change vehicles, consider whether the layout should leave more space than the current one needs.
