Word can only offer those sizes supported by the selected printer... the
info comes from the printer driver. Perhaps you have different capabilities
if your printer is spec'd for another market, but the Canon website lists
only the following sizes for that model:

Credit Card (2.13" x 3.39"), 4" x 6," 4" x 8," 5" x 7," 8" x 10," Letter,
Legal, U.S. #10 Envelopes

I'd suggest you contact Canon - or at least check for a different/updated
driver for the printer.