I think s-pen is a differentiator to some, not all, and if two phones are similar, many would pick the one with this 'feature'.
Otherwise I think the phablet that will 'win' will be the one that provides a large screen in the most usable package. The Note 2 nailed it for its generation - too big for many, but for those looking for a pocketable, single device with a larger screen - it was great and people got spoiled with the battery life.
So IMO - 5.7 to 6 is all doable as long as the phone remains easily pocketable, is no heavier than the Note 2, and gets competitive battery life. If any of the new phablets fails in any of these aspects, then IMO it won't be a mainstream success. That's why IMO the Z Ultra is DOA and will be a niche product. It's too big to fit into suit/pants pockets comfortable and likely has meh battery life.
As a Note 2 user, I can go a little bigger, but not much and no heavier as it already gets somewhat tiresome holding it for a while.
Well, you're speaking from your perspective. Fact is, SONY (and others) did the research and found out that phablets are big in poor countries. Why? Because people cannot afford phones and tablets. So, the bigger the screen the better. Those people don't care about a stylus. That is why the Xperia Z Ultra is not available here (now) and neither is the Samsung Galaxy Mega. Those devices are geared towards certain markets and the United States isn't one of them. Here, we can afford multiple devices. Over there, these phablets may be the only device people can afford. And, since they're phones, they are considered essentials...