It leaves the outer ones, while removing the middle 9. I'd not found one myself, or I'd test clicking on the outer rim of the door.
Still, if I could put the frame down, and then fill the "door" with any of the blast resistant xycorium blocks, I'd love it.
Size. 16 per interior block, as the same with buildcraft tanks (and thermal expansion, the number is consistent) the outsides are not counted, so it's less space efficient then the railcraft tanks, but while those max at 9x9x9 (if you set the server, otherwise smaller) these max at 12x12x12 outside dimensions, making it 10x10x10 on the inside, for 1000 contained squares, a total of 16,000 buckets. This lets them store a little more, and the valves work slightly better, with auto balancing.