Make sure to print at "100%" or "actual size" and NOT "scale to fit page" and set the paper orientation appropriately.
Unless otherwise noted, most pages are formatted for standard US letter-sized paper. Most (but not all) designs have enough margin on them to print successfully on A4 paper, but make sure it doesn't scale the image.
To make your mats more durable, you may wish to print on cardstock and laminate, or use spray-mount glue to adhere the pages to cardboard. Lamination also helps prevent smearing if these are printed on an ink jet printer.