The biggest I have seen is DeGroat's T8 90mm.
Not my pic but I was there when they were shooting this thing. It felt like the earth moved under my feet when it went off.
Here is a link to the owner's online photo album of it. http://www.armamentsales.com/photos/...ls.php?album=8
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer