WordPress: Make unique custom image size ONLY for featured image of a post

custom-featured-image-size.php

add_action( 'save_post', 'custom_featured_image_size', 10, 2 );
function custom_featured_image_size( $post_id, $post ) {
	
	$image_size_name = 'my-custom-image-size';
	$cpt = 'my-cpt';
	$width = 1600;
	$height = 1600;
	$crop = false;

	// Only for my CPT.
	if ( $cpt != get_post_type( $post ) || ! has_post_thumbnail( $post_id ) ) {
		return;
	}

	$featured_image_id = get_post_thumbnail_id( $post_id );

	$full_image_path = get_attached_file( $featured_image_id );
	$metadata = wp_get_attachment_metadata( $featured_image_id );

	// Check if image already exists, don't remake if it does.
	if ( ! empty( $metadata['sizes'][ $image_size_name ]['file'] ) ) {
		$existing_image_path = str_replace( wp_basename( $full_image_path ), wp_basename( $metadata['sizes'][ $image_size_name ]['file'] ), $full_image_path );
		if ( file_exists( $existing_image_path ) ) {
			return;
		}
	}

	// Make the new cover image and save to metadata.
	$image_data = image_make_intermediate_size( $full_image_path, $width, $height, $crop );
	$metadata['sizes'][ $image_size_name ] = $image_data;
	wp_update_attachment_metadata( $featured_image_id, $metadata );

}

your-theme-file.php

// How to use this in your theme
the_post_thumbnail( 'my-custom-image-size' );