Thumbnail Pictures from Shop are too small – I need really help

Hi i use the Theme azeria, its a great theme but i have really a big problem with the shop thumbnail pictures. My site is rouzbehhamzalufard.com Iam a artist and try to build myself a small Homepage with small shopsystem. Everything i do tht the thumbnail goes like in the Demo Shop, are wrong. I have try a lot of thinks but it dont want work. I have try to change the sizes in the function.php, after every step i have everytime also use the Plugin regenrate Thumbnail / pictures. But it helps not.

/**
     * Allow to rewrite defined image sizes from child theme
     */
    $default_image_sizes = array(
        'post-thumbnail' => array(
            'width'  => 450,
            'height' => 300,
            'crop'   => true
        ),
        'slider-thumbnail' => array(
            'width'  => 1900,
            'height' => 445,
            'crop'   => true
        ),
        'related-thumbnail' => array(
            'width'  => 450,
            'height' => 300,
            'crop'   => true

I have try to change it in custom css..but works not. Have try to change it in woocommerce.php , but it dont works.

* Add WC Image sizes to WP.
*
* @since 2.3
*/
private function add_image_sizes() {
$shop_thumbnail = wc_get_image_size( 'shop_thumbnail' );
$shop_catalog = wc_get_image_size( 'shop_catalog' );
$shop_single = wc_get_image_size( 'shop_single' );
add_image_size( 'shop_thumbnail', $shop_thumbnail['width'], $shop_thumbnail['height'], $shop_thumbnail['crop'] );
add_image_size( 'shop_catalog', $shop_catalog['width'], $shop_catalog['height'], $shop_catalog['crop'] );
add_image_size( 'shop_single', $shop_single['width'], $shop_single['height'], $shop_single['crop'] );
}

I have really no ideas, iam a beginner, i want to make it self, becouse learning by doing but now i must say i really need a littelbit help. I hope somebody takes a littelbit time and helps me out. Thx a lot…