The paper size refers to the actual sheet of paper that your publication will be printed on. Commercial printers usually refer to paper size as sheet size, and to the size of the finished publication page as the page size. Remember, the page size might be smaller than the sheet size. When this is the case, you or your print professional will trim the publication by using crop marks.
If you are preparing a publication for a commercial printing service, be sure to ask your printing service the exact size of the paper, or sheet, they will be printing on.