change featured image size / ratio

Hello everybody! I’d like to change my featured image size or ratio.
At the end I modified the functions.php like following, and it works:

set_post_thumbnail_size( 700, 366, true );
	add_image_size( 'twentyfourteen-full-width', 1200, 628, true );

But any attempt to include this feature in the child theme’s file has no effect.
Any help?

This is the code inside my funcion.php (child) file:

<?php

add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles()
{
	wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}

function my_child_theme_setup() {
	load_child_theme_textdomain( 'twentyfourteen', get_stylesheet_directory() . '/languages' );
}
function thumbnail_size_tweak() {
	set_post_thumbnail_size( 700, 366, true );
	add_image_size( 'twentyfourteen-full-width', 1200, 628, true );
}
add_action( 'after_setup_theme', 'my_child_theme_setup', 'thumbnail_size_tweak' );

?>