The cargo door aperture total size was about 5' 10" x 7'.0" and the normal para exit was via a removable door inside the forward cargo door frame. It was unlocked by two upper handles and lifted upward and inward in a lazy 'S' channel, if I remember correctly. It would be replaced after the jumpers had left.
It might be worth remembering that British forces, as well as American, jumped from these aircraft: the Normandy invasion, Operation Market Garden, the Rhine advance and Burma operations instantly come to mind.
The 'Dak'/C47 is very well-known in the UK to those interested in historic aircraft..