Something that large you're going to have to make multiple scans and digitally stitch them together, which is a ROYAL pain. The lighting refelections are never identical on multiple passes, and trying to rotate the images exactly right so they line up accurately is a virtually impossible task. There will almost always be a slight misalignment if someone looks closely.
My secondary scanner, a Fujitsu fi-5750c has a 12"x18" maximum image size, which is about the largest flatbed scanner on the market. There are larger scanners, but they tend to be floorstanding sheetfed scanners meant for scanning blueprints and are not appropriate for delicate materials. Also they are insanely expensive.
Even a good quality 11x17 or 12x18 flatbed is not going to be cheap. As I mentioned in this thread
, the original list price for my scanner was almost $9,000, now findable on the used market for $300-600... which is actually dirt cheap for what it can do. It doesn't have the optical resolution of today's modern scanners, at only 600dpi native resolution, but for most purposes I would use it for, it is more than adequate. It's lightning fast as a scanner as well... much faster than I would have expected.
You could always try using a digital camera, but that has a host of problems: keeping the document flat, even lighting, etc.
I have not found a good solution for oversized documents. Sometimes you have to make due with only scanning portions. I have some large vellum indentures that I would love to image, but their large size combined with the stiffness of the material, makes it all but impossible.
The 12x18 image size on the Fujitsu is large enough to accommodate 95%+ of what I need to scan, which is close enough for govornment work. Dealing with an 8.5x11 or 8.5x14 max image size was just far too constraining.