Ok I found out the solution.
Because, as I said, WooCommerce has it’s own hooks, first of all I’ve searched for a WooCommerce hook about registration.
I discovered the existence of a hook very similiar to the one used by WordPress: woocommerce_registration_errors. Obviously it works in the same way.
Then I simply replaced the WordPress hook with the WooCommerce one. Here the final snippet you have to insert in your functions.php file:
add_filter( 'woocommerce_registration_errors', 'my_registration_errors', 10, 3 );
function my_registration_errors( $errors, $sanitized_user_login, $user_email )
{
if ( strlen( $sanitized_user_login ) < 5 ) {
$errors->add( 'username_too_short', __( '<strong>ERROR</strong>: Username must be at least 5 characters.' ) );
}
return $errors;
}