Its possible to use a smaller scanner and an image
stitching program
to combine the individual scans into a single, larger raster image.
But please be careful not to do what I've seen on some websites (not
Bitsavers, I hasten to add). And that is to scan the schematic as 2
halves with no overlap. There is always some small bit cut off when this
is done and by Murphy's law, the bit you lose is essential (like giving
the order of connections of bus lines ot an IC, or part of a component
value, or....).
-tony