How can I get actual page size on monitor screen?

I want to have the actual page size on my screen, but the pages are bigger than 8.5 X 11. How do I get the actual page size to display onscreen, so I know what the actual page will look like?
Now the page display is 9.5 inches across, not 8.5.
How can I set the default page size to be 8.5 X 11?

The zoom percentage needed to get the actual size of the paper depends on many factors, including the operating system and the display’s size and resolution. A good down-to-earth way to determine this is to hold a letter-size sheet in front of the display and match its width with the width of the displayed page, assuming that the document’s displayed page has a width of 8.5 inches.

The resulting zoom percentage is the one needed to display the actual size of the document and can be set as the default for subsequent documents.