You can define Print Profiles related to the printer and to the type of file to print, so that documents are automatically printed to the correct size and to the right printer.
Select Custom paper size to activate the Width and Height box, then you can define a custom paper size.
TIP: The available paper sizes are related into your printer setup and there are only the paper sizes which your printer can handle.
Millimeters (mm) are used as dimension unit for the custom paper size.
Define the sheets to print with the current print profile