San-Juan Wolf is a giant who was a prisoner in Impel Down, until Blackbeard freed him.
But how does a man so huge, who is almost big as the Marine Headquarters, get put in the Level 6 of Impel Down? Also, when Blackbeard came, how does he get out?
It's not obvious, but there are a few possibilities.
They could have a devil fruit user working for the world government who's able to somehow get him in. For example, Law would be able to do this with his fruit by cutting Wolf up, and it's possible Blackbeard got Wolf out by absorbing him into a black hole and using "Liberation" to set him free outside.
The other obvious possibility is that there is a second, much larger entrance to Impel Down underwater, and Wolf was brought in by a giant submarine.
It could be either of these, or something else entirely, it seems ridiculous, but in the world of One Piece almost anything is possible.