Slides from Slate magazine's article on the quest for a 40-foot anaconda, although the researchers would have accepted a 30-footer. Unfortunately, they found neither, and the claimed photograph of a 120-footer shown here looks like a sandbar to me.
COMMENT: The late Smithsonian herpetologist George Zug accepted a 38-foot claim, although some newer authorities reject this report (the snake was not preserved). (I use English measurements here because of the iconic nature of the 30-foot monster.) It's been argued the prehistoric 43-foot Titanoboa represented an upper bound for snakes, because at some point the nerve impulses travel too slowly to control the complex body movements.
Thanks to Dana Stabenow for first posting this link on Facebook.