Check your phone's print service options. If it uses a newer version of Android, there may be a "Default Print Service" option; ensure that is set to Off. The Default print service may cause all print jobs to come out as card-sized small-sized, forcing the printer to try printing to small-size paper when using the Canon Print Service or MOPRIA.
Once the Default Print Service has been disabled, verify in the print window that the paper size and media type selected are the ones you wish to use (from your description, "Letter" size and "Plain Paper" type). The expanded print options may be found by tapping the downward-pointing arrow at the bottom of the printer selection and print preview screen.
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