I was about to mention the legacy project photo until I saw Lexla's link.
9.6 x 33.8 metres, and airplane hanger for a camera. If not the largest, then certainly pretty close.
Here's another link for it; http://www.legacyphotoproject.com/index2.php

There's also the Nascar photographer Rick Graves who does 7 x 300 foot prints from a continuous length of film.
http://rickgravesphotography.blogspo...a-alabama.html