Well I'm not sure if its the biggest, but theres this thing called the gigapixel project, they use a camera that is over 1 gigapixel (like 1000 times better then a camera phone), and the prints they get a humongous. I can't remember the website, but you could google it
The gigapixel camera is a 9"x18" large format camera; definately big, but not as big as some people are using these days. This 96"x54" camera makes it look sub-miniature. Although I doubt anyone ever built a big enough enlarger to handle an 8'x4.5' neg.
The project's name is spelled gigapxl, and its website is http://gigapxl.org/ ;-)
I think it might hold the record for the most pixels in a single exposure (don't quote me on that, though), but I don't know what the largest print is.
I may be making this up but didn't Kodak use to have an ongoing display in the terminal at Grand Central Station of huge prints (like 8' x 24') made from 35mm Kodachrome?