Hi In \src\PrestaShopBundle\Form\Admin\Product\ProductInformation.php in line: 161, 165. Try change value counter and max
I haven't actually tested it.
'options' => [
'attr' => array(
'class' => 'autoload_rte',
'counter' => 6000,
),
'constraints' => array(
new TinyMceMaxLength(array(
'max' => 6000
))
),
'required' => false
],