Apple Computer must be running out of ideas if they think that the a 6.5" screen size is what people want, based on the rumoured iPhone XL. It certainly has not learned any lesson from the lacklustre sales of the Apple Watch.
Here is how screen size plays out its role. Basically the world of communication devices can be divided into 3 "screen sizes", which I simply call large, small, and tiny.
We use large screen sizes where we need to display a lot of information on one screen. Typically this is when we deal with spreadsheets and visual presentations, and sometimes even text documents. Many of these documents have to do with business and education. That is why we have desktop and laptops. Even if we can shrink the computers into a palm-sized gadget, we still need to have a larger screen size. So desktops and laptops are not going to be replaced by smartphone size gadgets anytime soon; or rather, the screen size that I call "large".
A small screen size like what we have on a smart phone is used where we only need to show limited information on one screen to do what we want to do (ever tried to work with a spreadsheet on a smartphone?). There is a compromise between portability and ease of viewing, and most people settle for a screen size between 5" to 5.5". Apple XL with a giant 6.5" screen will not be able to convince the world to shift to this size unless we all grow larger hands and big pockets become a fashion trend.
Finally, the tiny screen. Obviously Apple Computer has not learned that the reason Apple Watch failed to replace many of the things we do on a smartphone is because we simply need the larger real estate for the things a smartphone is designed for. A tiny screen is useful only for apps with very, very limited information to display. We cannot use it for email, and for social media. We cannot use it where we need a keyboard to enter text.
If you are buying a smartphone today, just choose the screen size you are comfortable with. Definitely a 6.5" screen size is going to be awkward and would probably only appeal to you if you are choosing between having to use reading glasses or splurging on a bigger display.