According the keepers of human history over at the Guinness World Records, the largest circular pizza ever baked weighed was made in Norwood, South Africa by Norwood Hypermarket on December 8, 1990. It weighed 26,883 pounds.
The data is a bit sketchy, but here are relevant numbers: The pizza measured 122 feet, 8 inches in diameter, weighed 26,883 pounds, and contained 9,920 pounds of flour, 3,960 pounds of cheese, 1 763 pounds of mushrooms, 1,984 pounds of tomato puree, and 1,984 pounds of chopped tomatoes.
Is there video? Yes, there's video:
Video: EPIC 37-Meter Pizza