Dimensione immagine personalizzata

if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'new-size', 500, 300, true );
}
add_filter('image_size_names_choose', 'my_image_sizes');
function my_image_sizes($sizes) {
$addsizes = array(
"new-size" => __( "New Size")
);
$newsizes = array_merge($sizes, $addsizes);
return $newsizes;
}