change featured image size

hello there how can I change the “featured image” size?

I found this in the punctions.php theme file:

if ( ! function_exists( 'sparkling_setup' ) ) :
/**
 * Sets up theme defaults and registers support for various WordPress features.
 *
 * Note that this function is hooked into the after_setup_theme hook, which
 * runs before the init hook. The init hook is too late for some features, such
 * as indicating support for post thumbnails.
 */
function sparkling_setup() {

	/*
	 * Make theme available for translation.
	 * Translations can be filed in the /languages/ directory.
	 */
	load_theme_textdomain( 'sparkling', get_template_directory() . '/languages' );

	// Add default posts and comments RSS feed links to head.
	add_theme_support( 'automatic-feed-links' );

	/*
	 * Enable support for Post Thumbnails on posts and pages.
	 *
	 * @link http://codex.wordpress.org/Function_Reference/add_theme_support#Post_Thumbnails
	 */
	add_theme_support( 'post-thumbnails' );

  add_image_size( 'sparkling-featured', 750, 394, true );
	add_image_size( 'tab-small', 60, 60 , true); // Small Thumbnail

	// This theme uses wp_nav_menu() in one location.
	register_nav_menus( array(
		'primary' => __( 'Primary Menu', 'sparkling' ),
		'footer-links' => __( 'Footer Links', 'sparkling' ) // secondary nav in footer
	) );

	// Enable support for Post Formats.
	add_theme_support( 'post-formats', array( 'aside', 'image', 'video', 'quote', 'link' ) );

	// Setup the WordPress core custom background feature.
	add_theme_support( 'custom-background', apply_filters( 'sparkling_custom_background_args', array(
		'default-color' => 'F2F2F2',
		'default-image' => '',
	) ) );

  // Enable support for HTML5 markup.
  add_theme_support( 'html5', array(
    'comment-list',
    'search-form',
    'comment-form',
    'gallery',
    'caption',
  ) );
}
endif; // sparkling_setup
add_action( 'after_setup_theme', 'sparkling_setup' );

so I copied and then modified it in my child theme “functions.php” file,
then I modified:

add_image_size( 'sparkling-featured', <strong>750, 394</strong>, true );

I added all this code to my child functions because adding only “add_image_size( ‘sparkling-featured’, 750, 394, true );” doesn’t work.

so can you suggest a better way to reduce the featured image size without the superfluous parts? just to understand for the next time 🙂

sorry but I’m really new in this!

Thank you in advance