Photomerge for Large Originals

When you have an original that is too large to scan in one pass scan it in 2 or more then:

Photoshop CS4
File>
Automate>
Photomerge>
you can choose from 2 or more files or a folder
mainly choose the first choice: Auto
check -blend images together.

Photoshop will run through the actions
and leave you with 2 layers which you can look at to see the blends.
Mostly it works great.

If you like > Merge Layers (Control E)

You may have to crop in and distort as needed for squareness. (Control T holding down shift and or option)