woocommerce_thumbnail_size │ filter-hook │ WC 1.0

Usage

add_filter( 'woocommerce_thumbnail_size', 'wp_kama_woocommerce_thumbnail_size_filter' );

/**
 * Function for `woocommerce_thumbnail_size` filter-hook.
 * 
 * @param  $string 
 *
 * @return 
 */
function wp_kama_woocommerce_thumbnail_size_filter( $string ){

	// filter...
	return $string;
}

$string

-

Where the hook is called

$thumbnail_size       = apply_filters( 'woocommerce_thumbnail_size', 'woocommerce_thumbnail' );

Where the hook is used in WooCommerce