Путь - /public_html/src/PrestaShopBundle/Form/Admin/Product
файл - ProductSeo.php
$builder->add('meta_title', 'PrestaShopBundle\Form\Admin\Type\TranslateType', array(
'type' => 'Symfony\Component\Form\Extension\Core\Type\TextType',
'options' => [
'attr' => ['placeholder' => $this->translator->trans('To have a different title from the product name, enter it here.', [], 'Admin.Catalog.Help'),
'counter' => 70], - меняем длину на свое значение
->add('meta_description', 'PrestaShopBundle\Form\Admin\Type\TranslateType', array(
'type' => 'Symfony\Component\Form\Extension\Core\Type\TextareaType',
'options' => [
'attr' => ['placeholder' => $this->translator->trans('To have a different description than your product summary in search results pages, write it here.', [], 'Admin.Catalog.Help'),
'counter' => 160], меняем длину на свое значение