Unless 4.5" x 8" is one of the sizes offered by your printer driver, you're
probably using a Custom paper size. Some printers can handle custom sizes,
but you have to figure out where the printer expects you to feed the paper.
If that doesn't work (for example, if the printer expects the paper to be on
the right or left, but the paper guides converge in the center for small
paper sizes), then you may have better luck setting up your small document
on a standard  paper size (such as Letter) and adjusting the margins to put
the print area where you will be feeding the small paper.