Canon mx860 - Manual (Page 451)

4. Click Specify....
Select Stitch Assist for Document Size, then specify the scanning resolution as required.
When setting is completed, click OK.
Scan Settings Dialog Box (Photos/Documents)
5. Return to the Scan/Import window and click Scan.
The Stitch-assist window opens.
6. Place the left half of the document face-down on the Platen.
Page 451 of 997 pages Scanning Images Larger than the Platen (Stitch Assist)