Custom Single Post Template for Posts??

Same here. can’t find info on a unique page template for a specific page in wordpress.

one method that comes to mind right now would be to use the body_class function and make the changes directly in the CSS file, by adding the dynamic unique page-ID class generated.

https://www.nathanrice.net/blog/wordpress-2-8-and-the-body_class-function/

something like this:
body.page-id-3 .YourDIV {width: XYZpx;}