I have an A3 colour scanner, but my artwork is often larger than that in size.
To capture the full image, I do four scans with each scan area overlapping. I then use Microsoft ICE (Image Composite Editor) which is free, to then assemble and join the four images. It's an automated process and works very well. Often the peripheral edges dont always align, so you may may need to crop around the edges a few pixel rows, else scale (stretch) or copy the adjacent visible pixels to the missing edge row in Photoshop or whatever editor you use.
My scans are 1200 dpi and the joins to my eye are seamless almost 100% of the time, despite the "edge" issue. But then again my artwork is abstract, so it may be a more forgiving genre.