When I scan in Big watercolor pieces (this ones 22" across) I end up battling with the edges. Big pieces in which I've used lots of water warp and buckle. That shows up on the sides in the scan as ugly dark splotches. I've started to actually IRON them and that seems to help. They still are curving when I put them in the scanner, but if the edges are sharp and flat I can put a large book on the scanner and turn off the room lights (important) and get a decent scan.
Also, The foreground is a tiny brush and ink and the back is pencil. A trick I learned from watching Stacy.