Canon mp620 - Manual (Page 388)

Select Stitch Assist for Document Size, then specify Scanning Resolution as required.
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.
7. Click Scan.
The left half of the document is scanned and appears in the Stitch-assist window.
8. Place the right half of the document face-down on the Platen.
9. Click Scan.
Page 388 of 815 pages Scanning Images Larger than the Platen (Stitch Assist)