At the widest point, the pressure hull is slightly less than 5 meters in diameter and tapers to both ends. In the video, Chatterton enters through the blast hole amidships and moves forward to the torpedo room.
This hatch leads to the aft torpedo room, which also served as crew berthing. In the event of damage, these hatches would be closed to seal off the watertight compartments. Whatever happened to the U-869 must have been immediate and catastrophic, as all the hatches appear to be wide open.