When creating a document for printing, the best size for a sheet would probably be that of an A4 page.  On the other hand, dashboards are usually better off having sheets whose sizes correspond to common screen resolutions to allow users to see everything without having to scroll around.

To bring up the Page Properties window, navigate to Format->Page Size, or right-click an empty area on the sheet itself.

 

There are several options in the Page Properties window:

Option	Description
Auto-Size	An ‘unlimited’ sheet with no boundaries
Pre-Defined Size	Assign one of pre-defined size values
Custom Size	A custom size (in pixels)
Page Orientation	Portrait or Landscape page layout
Set As Default	Uses the entered values as default values
Apply To All	Applies defined properties to all sheets