When you first add a page designer app to a dashboard, it'll automatically open the new page designer app's settings. You can select:

• Table: the table containing the records you want to turn into pages.
• Record size: the size of the page you want to design. By default, it's set to Letter (8.5 x 11 in), but you can set it to another preset (like A4 or Business card) or set your own custom dimensions.
• Record orientation: either Portrait or Landscape.