// Filters the size on media:content tag. Defaults to 'full'.
add_filter( 'wp_feed_post_thumbnail_image_size_full', function( $size ) {
	return 'large'; // Return any registered image size.
}, 10, 1 );

// Filters the size on the media:thumbnail tag. Defaults to 'thumbnail'.
add_filter( 'wp_feed_post_thumbnail_image_size_thumbnail', function( $size ) {
	return 'medium'; // Return any registered image size.
}, 10, 1 );

// Filters the featured image attachment post object.
add_filter( 'wp_feed_post_thumbnail_image', function( $thumbnail ) {
	return ''; // Return an empty string or another attachment post object.
}, 10, 1 );

// Filters the array of attachment post objects. Defaults to featured image post object if exists.
add_filter( 'wp_feed_post_thumbnail_images', function( $images ) {
	$attachment_id = '123';
	$images[] =  get_post( $attachment_id ); // Additional attachment post object.
	return images;
}, 10, 1 );

// Filters the title on the media:title tag. Defaults to attachment title.
add_filter( 'wp_feed_post_thumbnail_title', function( $title ) {
	return 'Override title'; // Return any plain text.
}, 10, 1 );

// Filters the text on the media:description tag. Defaults to attachment description.
add_filter( 'wp_feed_post_thumbnail_description', function( $description ) {
	return 'Same description for all images'; // Return any plain string.
}, 10, 1 );

// Filters the name of the author on the media:copyright tag. Defaults to attachment author.
add_filter( 'wp_feed_post_thumbnail_author', function( $author_name ) {
	return 'Matt'; // Return any plain string.
}, 10, 1 );