Setting the page size of your map layout
Since a layout is the arrangement of map elements of various types on a page for printing, one of the obvious initial steps in building your layout is to set the desired page size using the Page and Print Setup dialog box. By default, when you create a new map in ArcMap and choose Blank Map, the layout page dimensions are set to your default printer's page size.
Steps for setting the layout's page size
1. Click File Page and Print Setup on the main menu to open the Page and Print Setup dialog box.
2. You can set your layout page size as shown here.
3. By default, the page size of your layout for a blank map will be set to the printer's paper size (for example, 8.5 by 11 inches).
4. Note:
5. You may want to change the printer to a large format printer (plotter) in your organization. Use the Name drop-down to set the current printer.
6. To set your own page size, uncheck the Use Printer Paper Settings check box to set the page size of your layout. If you uncheck this option, you can choose any page dimensions you want, but the printer name will not be saved with the map document (.mxd).
7. Set the page size by typing the preferred page dimensions into the Width and Height boxes.