The biggest radiographs I've ever personally handled have been 14 x 51 inch (0.36 x 1.3 m) films of an entire human spine.
However, these are pretty small potatoes compared to some of the work by photographer Nick Veasey, who has X-rayed backhoes and entire buses full of people. Check out this shot of a Boeing 777 and hangar, which may be the largest radiograph ever taken.
Besides these enormous objects, his online portfolio also includes a lot of other cool stuff, such as bats, spiders and the perennial male favorite: looking inside ladies underwear.
(via Movin' Meat)