Increasing character limit on a form field

Hi,

That page is not in HikaShop but in Joomla. You can't edit it via the Display->Views menu of HikaShop so that's normal you don't see your changes.
That form is in the file components/com_user/views/register/tmpl/default.php

That is not a hikashop page but the joomla registration page. You would have to edit the view and add some javascript in order to remove the username field and set it as a hidden input and set its value to the email input when the form is submitted.

In HikaShop, if you change the "registration" option of the configuration to simplified registration, the username field will be removed automatically and the email will be used as username.

Indeed. But note that it won't change the joomla user registration screen. It will only affect the hikashop registration screen.