How can i see in Dashboard or reports, which customer prefers which brand and buys which size at most?
There is Aheadworks extension called Advanced Reports:
They provide you 13 reports, plus you can buy additional pack of 10 more reports. You should check its capabilities if it matches your needs. You can find demo link there, too.
I found this extension too, but they said it´s not possible to have a report with "customer - brand" with their extension.
There is one extension called Wisdmlabs Wishlist Notification for Magento 2.0.
It's a free extension that notifies the admin via email whenever any customer adds a product to his/her wishlist.
Also, you can view any customer's entire wishlist as well, which will help you determine which products are in demand and accordingly modify the prices and offer striking deals.