Double-click the vertical ruler to open the Page Setup dialog. In the
Margins tab, set the four margins to 0. In the Paper tab, set Paper size: to
Credit Card. You can choose either Portrait or Landscape. When you click OK,
Word will offer to "fix" the margins. If you let it do that, it will set
them to the minimum size that works (in theory) with your printer.
After that, it's a matter of choosing a font size that will do what you
want. I would try pressing Ctrl+A (select all), then using Ctrl+[ and Ctrl+]
to decrease/increase the font until I got a size that fits and is still
legible. The latter is debatable. ;-)