Saw the shell in Etajima yesterday next to the kaiten. Doesn't look that big. Kind of underwhelming after Kancolle comics.
Naval gun shells are extremely dense. The USN went out of their way to make them even heavier for their size since this apparently increases penetration even further. That's why the 16"/50 Mk 7 has near-equivalent armor penetration to the 46 cm/45 Type 94 despite being less than 75% its weight.
I remember hearing somewhere that there was a life-sized mockup of the 20" (50.8 cm) shells planned for HMS Incomparable on display at a certain museum, but my Google-Fu doesn't seem to bring up anything. I'd be curious to see how it scales with a human.
