The |pgfpages| can do all sorts of tricks with pages. The most important one for printing \beamer\ slides is the following command:
This says ``Resize all pages to landscape A4 pages, no matter what their original size was, but shrink the pages by 5mm, so that there is a bit of a border around everything.'' Naturally, instead of |a4paper| you can also use |letterpaper| or any of the other standard paper sizes. For further options and details see the documentation of |pgfpages|.