Unless your paper supports this paper size (offers it as a size in the Page
Setup dialog), you may have to fudge. Some printers handle custom paper
sizes fairly well, provided you feed them where it expects to find them, but
if you can't feed the paper where the printer expects to find it, then
you'll have to set up your page on the next larger size the printer does
support and adjust the margins accordingly (that is, add the difference
between the paper sizes to the margins you were already going to use).

Sometimes it's easiest to figure this out by drawing around your small paper
in the position where you'll have to feed it relative to the larger paper
size.