This is something you need to work out with the printer. If your printer
will actually print on that size (most won't), set the page size and go for
it.

More usual is to print onto a larger sheet and cut it up. You can use a
table to create crop marks.

I take it these are individual cards?

As Jezebel indicates, printer drivers are often poor with custom paper
sizes, so if your printer is not configured with these sizes as standard,
you have to fool the printer. The printer will either adjust to paper sizes
from one side or both sides. If the former pin a card to a piece of regular
paper (A4 or US Letter as appropriate) at top left. If the latter pin top
centre. With a ruler, measure the exposed paper and adjust the margins to
reflect what you see. When happy, save the result as a template and use to
create the documents.