Hi,
It's doesn't use a view. It a javascript library which generates the popup. And the width and height of the thumbnails is dictated by PHP code in the controller of the add to cart system so it's not a view either.
Now, we do have a setting to be able to increase the size of the product thumbnail in the notification. For that, you need to add an entry with the namekey addtocart_popup_image_size and in the value the size in px you want in the table hikashop_config in your database via your phpmyadmin.
Then, you'll have the image in a bigger resolution. And you'll have to add custom CSS on your website to adapt the popup.