FWIW, index cards are usually 5" x 8". If your printer doesn't offer this
paper size or allow you to create it, you may still be able to create it as
a custom size in Word and print it using the manual feed tray of your
printer. If not, figure out how you can feed it through your printer and
adjust the margins on a Letter-sized document accordingly. That is, figure
out where on a sheet of Letter paper the print area has to be to print on a
5x8 card fed whatever way your printer requires, and increase the margins on
the appropriate sides to get the text area down to where it will fit on the
card.