I've seen the head of a blue cat (Upper River, above Hannibal, MO) that measured about 18 to 20 inches in diameter. It was only the head, the result of cleaning by (probably) a commercial fisher.

Back in the early '60s a hard hat diver went down to inspect bridge piers on the bridge at Quincy, IL. He returned to the surface with tales of two eight-foot long catfish he had seen and which weren't happy with him being there.

LaSalle reported catfish and sturgeon as long as his canoe, and I've had some personal experiences on The River that lead me to believe that not all of the BIG fish are gone.