No Full Width Option but full width.php

Hello WP users,

My client’s theme does not have a Full Width option for the page template. I’ve read some articles on how to create a full-width template and doesn’t look too difficult, but I need to access their server and they have no logins for me.

I noticed though in the Appearance editor that there’s a “full width.php” file and code. Would this imply that I should be able to make this option available when I’m creating a page? First time I haven’t seen the Full Width option in a theme before. I just have Home page and Default which includes a sidebar.

Many thanks.