The trouble I have with scanning large documents is the "lip" around the scanner's perimeter. If you can remove the lid and bezel, you can get the map to lay much flatter, eliminating the lift at the edges and allowing you to scan the center of the page easily.
Make each scan with a little bit of overlap, then move them all into a single Photoshop document. Select all of the layers, go to edit > Auto Align layers... Experiment with the different projections to get one that works. Usually collage or reposition work well for me.
Howdy people. I thought I'd resurrect this thread to see if there are any new methods or tools for scanning large images. I'm currently hand-drawing a lite challenge map on an A3 page (297x420mm; 11.7x16.5"), and don't have a large enough scanner. So, does anyone know if the art of scanning large pages has moved on since 2010?