The sheets info area was about 14"x60". I did them in 14"x8" increments and then pasted them together. It took many, many hours (and lots of RAM)! There are larger flatbed scanners out there, in fact, one of my friends is getting one to scan artwork. Maybe by late spring I can use it.
Copy Ship and Mail in Baker City has a very large format copier that might be helpful for maps.
Another thought, how about setting up a quality digital camera on tripod to do the images. I've done some art work photos of 20x30" art with my Canon Digital Rebel, that print beautifully at that size. You have to be very careful to get even lighting, a handheld incedent light meter helps a bunch with this.