Hi,
If you want the change the size of the thumbnails on all the order/cart products listings on both backend and frontend, you can add an entry with the namekey cart_thumbnail_x and the value you want for the width and another entry with the namekey cart_thumbnail_y and the value you want for the height in the hikashop_config table via your PHPMyAdmin.
That's the best as it avoids having to add an extra override.
Otherwise, if you really just want it for the order products listing in the backend, you can edit the file order / show_products.php via the menu Display>Views and change the numbers on the lines:
$width = (int)$this->config->get('cart_thumbnail_x', 50);
$height = (int)$this->config->get('cart_thumbnail_y', 50);