Our drupal developer left company, so now I'm learning it )

If I open any page editor in admin panel, I see 'custom template' selectbox. In file structure I have some page-- and node-- files with similar names. 
For example in select box there is item 'Full width template'. And there are node--custom--fullwidth.tpl.php and page--custom--fullwidth.tpl.php in file structure.

So I need to create one more custom template but I can't find it in admin panel. How to do it, tell me please. 

Thanx!