How to set Fixed width on page

Hello, I have Catch Themes Pro. On a page, I want the fixed width to be 1400px - without affecting other pages that use the template full width no sidebar I made a child theme, copied page-fullwidth.php to page-fixed width in my child theme directory (gave description of page fixed width no sidebar) and now want to set the fixed width How do I assign it body class or What do i I need to do Thanks