Hello Damundson,
If you are trying to print from an iPhone, the printing option are limited and will usually not have an option to select your paper size. If you are trying to print photos and need to select paper sizes and paper types, I would suggest downloading and installing our free Canon print Inkjet/Selphy app from the App Store.
The app will let you print photos, documents and use the scanner. When using the option for photo print, you will see a gear for settings that you can tap on to select your paper size and paper type. When opening and closing the rear tray, a window should come up asking to register what size and type of paper was just loaded and you can follow the steps using the link provided HERE.
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