You can print on just about anything in Word provided your printer will
handle it. What this often comes down to is how well the printer handles
custom sizes, and usually some trial and error is required to figure out
where you'll need to feed the card and what you then need to do to get Word
to print in the corresponding location. For a one-off, it's usually easier
to write by hand, but if you have many to do, it may be worth the effort.

Note, however, that in certain instances (thank-yous for wedding presents or
memorial gifts, for example) writing by hand is de rigueur (to my grief, as
my hand cramps up with very little effort, and my late father was much
beloved and remembered by many with cards, letters, and memorial gifts).