Not a lot as the rear seats don't fold down flat, and you have the bin/ loudspeaker attached to the door, so maybe only be approx. 6’.
 
It all depends what you are trying to put inside the vehicle, and which seats you are prepared to remove.
I have managed to transport 2m40 lenghts of material in my D1, such as packs of wooden flooring, To do that I removed the left hand seats both back and front.
At the end of the day, all you need to work this out is a tape measure.
 

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