The rumour that Apple is working on a 6.5 inch variant of the iPhone X (possibly for next year) makes a lot of sense.
When you consider that this year’s 5.8 inch screen device is similar in size to the iPhone 6/7/8 then it follows that a larger version would be roughly equivalent in overall dimensions to the current Plus models.
Sure enough, the overall unit size of the current Plus phones are roughly 6.5 inches. A larger X should, therefore, be a little larger.
This would also tie in with the suggestion that the X is the future of the iPhone implying that the existing models will be phased out.
This year was an anomaly with Apple releasing three main devices. I would fully expect them to get back to releasing two with an updated X and a new X Plus.
And this is without touching on the obvious angle of competition.