Canon 3300B002 On-Screen Manual page 390

Scanning Images Larger than the Platen (Stitch Assist)
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.
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