This option has been given to you for addition of New Job Size. For addition of New Job Size you will have to go through this path; Job/Customer Master>Job New Size> New

As you access through the path a window for addition of New Size will open there in which you would write desired size, paper size and Paper Feed. After this you need to press Enter for its updation. 

If you are willing to change or delete existing job size, you need to press Enter on desired sized that exist there . After this you would get active options for changing and use Ctrl+F8 for deletion.