How do I change the template specifically for single posts?

No custom template is necessary.

Copy sidebar-primary.php to your child theme. Change first line to:

if ( is_page_template( 'templates/full-width.php' ) || is_single() ) {

Go to Customize -> Additional CSS, and add:

.single-post #main {
  width: 100%;
  float: none;
} 

Done.