I scan oversize documents and photos in sections with a regular flatbed scanner then stitch them together with Photoshop's panorama feature.
My scanner, Canoscan 8800F only scans negatives up to about 3 inches wide but I was able to scan my father's old 4x5 negatives with it. Just make 2 scans. Make sure more than half the picture overlaps then import into Photoshop via FILE > AUTOMATE > PHTOMERGE... The computer does most of the work.
All the pictures in this gallery came from my father's old negatives. All but the first one came from 4x5 negatives. All of them were scanned in two passes then stitched with Phtoshop.
Works for me!