Have a look in your Manual for Compatible Media Types, my canon (MP510) has a table that shows paper sizes that can be used and also states:
"For paper to be loaded in the Auto Sheet Feeder, you can also set custom paper sizes manually. Specify custom paper sizes within the following range.
Minimum size: 2.13 x 3.39 inches / 54.0 x 86.0 mm
Maximum size: 8.5 x 23 inches / 215.9 x 584.2 mm "
I should think you could use Word to set the custom page size, or maybe the Envelope or Label setting, don't know if you should use Portrait or Landscape as didn't look that far into it.