Hi,
Yes that's the file where you want to change that. However, it's better to do it with a view override via the menu Display>Views.
You'll want to change the line:
<div class="hkc-xl-4 hkc-lg-6 hikashop_product_block hikashop_product_edit_description"><div>
For example, you could set the hkc-xl-4 to hkc-xl-8
And the line:
<?php echo $this->editor->display(); ?>
which you could replace by something like that:
<?php echo $this->editor->width = 1100; $this->editor->display(); ?>
where 1100 is the width in px that you want for the editor.