Full-width featured images

I am creating a new child-theme for a client, and it’s looking good EXCEPT that I can’t get the blog post featured images to extend full-width.

I can code a little and assume the answer lies in creating a child-theme version of content-blog-image-large.php and related files areound this code:

if( has_post_thumbnail() ) {
			if( get_theme_mod( 'spacious_site_layout', 'box_1218px' ) == 'box_1218px' || get_theme_mod( 'spacious_site_layout', 'box_1218px' ) == 'wide_1218px' ) {
				$featured = 'featured-blog-large';
			}

Can you help with code modifications for that?
Thanks very much.
Caron

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