How big of things can you put up your butt?

There are pictures, floating around the internet, of adult men or women with big objects inserted up their butts. Objects the size of a fist and wrist. Or a Coke can. The Department of Homeland Security recently issued a warning about terrorists possibly trying to hide explosives or bombs up their rectums. I would think that an entire pound or half-kilo of plastic explosive could be shoved up there. But don't try this at home. You could hurt yourself. Nobody said it was safe or healthy. I'm just saying it's possible.