Internally they are rather like folding roll-film cameras. A couple of sprung studs for the film spools, with one driven for film advance. The film spool chambers are separated by a wall from the exposure area. Since there is no need for a pressure plate the risk of lateral light leak is higher than with a folder. And you need at least one red window set for the format you are using. Sizing is driven by the film width and the image dimension, with the pinhole quality dictating the depth of the camera for the coverage. I'm not sure actual measurements will be that much help. My 6x9 Zero Image could use a little more inter-frame spacing in fact.