Recently I was told by a friend that most of watching users prefer larger tablet (more than 8 inch with ratio 80 /20). Because it is a big difference I`am wondering if there is any research or statistics that actually gives a statistical preference
I haven't seen statistics on either fact. What I know as a person who has both tablet sizes and with many friends and colleagues with both (plus smartphones and "phablets") is they it doesn't matter. For a tv show, people will watch it on anything. The bigger screen is better, but there's a cost-benefit analysis that determines if time put into getting that larger device (or even connecting to a TV) is worth the effort over just watching it on the smaller screen.
Personally I love watching stuff on a tablet because mine has an amazing OLED display. Picture quality is way better than my tv. But there is never a case where I'd rather watch something at the end of the day on the tablet instead of the TV, where I could lounge back on the couch and relax, without having to hold a device directly. And I know that sentiment is widespread.