Geraldo Rivera is a well-known TV journalist and talk show host. I guess the "at large" part in the title of the show is supposed to suggest that Geraldo does his show from a variety of locations. Since Geraldo tends to be quite controversial and divisive sometimes, I guess the implication could also be that "he is a bit like a criminal that has been set free on an unsuspecting public".
![Surprise [:O]](/emoticons/emotion-3.gif)
I have never watched the show, though, so I'm really just guessing.