In the Master Pages tab, you can view changes to a form’s basic page layout, including the page size. When you select a master page in the Hierarchy palette, you can change its basic page layout properties in the Master Page tab of the Object palette.
Tip:
Content areas define the region in which objects can be placed on the pages of a form. If you are creating a form that contains subforms set to flow content, it is best to define the size and position of all content areas before you add other objects to the master page.

To specify the size of a master page