This will set the page to display at full scale. This would be similar to the scale at which the map would appear if the equivalent map document were exported from ArcMap and then printed. This scale will be very close to correct if the user's display DPI corresponds to a physical measurement. This would be set by the following procedure: in Control Panel-Display dialog, click Settings tab, then Advanced button; in the dialog that opens, click General, then use the drop-down list to choose Custom setting...; in the setting dialog, drag the ruler so that the scale corresponds to a physical ruler. In practice, few computers are adjusted this way, but instead are left at the default 96 DPI. Setting to a custom DPI may cause some dialogs and web pages to not display as designed.

Reference

PageLayout Class

PageLayout Members