Add image size

Hi, Sorry for my english and thank you very much for your work and all your themes. I'd like to know how to add a new image size on free catch responsive theme. I tried this code (and other) but it didn't work, the servor failed :

if ( function_exists( 'add_image_size' )) {
	add_image_size( 'header-size', 1200, 300, false );
}
add_filter('image_size_names_choose', 'my_image_sizes' );
function my_image_sizes($sizes) {
	return array_merge ( $sizes, array(
	'header-size' => __( 'Dimensions de l'image d'en-tete'),
	));
}

I wrote it in the functions.php file of my child theme. My aim is to have a new image size for my header picture. I don't know if it is necessary to add code in the header.php file. Thank you!