It was clear from his pics that it was THAT SMALL.
But he did not go into details about the issues you mentioned. He did mention that shielding was a problem but, if my memory is accurate, evidently not as terminal a problem as some might suspect. Again, he didn't elaborate.
My impression was that he treaded secrecy lines very carefully though quite confidently and almost nonchalantly.
There are a few rumors that some of the ET craft use nuke engines--though that seems to conflict with the zero-point energy engines most supposedly run on.
Again I guess I could see a motive force that small, but for power it would have to be coupled to dynamos. At the time, superconducting generators would not have been available, so he must have been talking about outboard devices.
Intriguing.