override single post featured imgae size

add_filter( 'wp_get_attachment_image_src', 'change_featured_size', 99, 4 );

function change_featured_size( $image, $attachment_id, $size, $icon ) {
    if ( 'jnews-featured-750' === $size ) {
        $full_size = image_downsize( $attachment_id, 'full' );
        if ( $full_size ) {
            return $full_size;
        }
    }

    return $image;
}