Canon MG3500 Series Online Manual page 144

Important
When you use a custom page size, correctly set the size of the paper loaded in
the machine in the Custom Paper Sizes dialog.
If an error message is displayed even though paper of the same size that was
specified is loaded, uncheck the Detect the paper width when printing from
computer check box.
3. Apply the settings
Click Apply and when the confirmation message appears, click OK.
The machine operates with the modified settings hereafter.
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