The diameter is 1150 mm. The 12 weighs 530 kg dry and the 21 weighs 590 kg dry. The bore is 130mm and stroke 150 mm. That's all I can find at the moment as I'm busy this time of year, but try this link and I'll bet something of interest is in the book. It's to a pdf I have The Nakajima Aircraft Engines book.
The Nakajima Aircraft