iPhone X Plus rumored to be same size as the iPhone 8 Plus
Citing information from a "reliable supplier information source", Japanese site Macotakara claims that the upcoming 6.5-inch iPhone X Plus model is roughly the same size as the current 5.5-inch iPhone 8 Plus.
Image credits: Macotakara
As you can see from the pictures above, the 6.5-inch iPhone X Plus is slightly bigger than the current 5.8-inch iPhone X and the 5.5-inch iPhone 8 Plus. This feat is achieved by using an edge-to-edge display with minimal bezels. The iPhone X Plus is expected to be slightly thicker due to the "mounting form of the rear camera".
The successor of the iPhone X will retain the same display size, but could sport a bigger camera sensor and lens.
As for iOS 12, Macotakara believes Apple will add support for horizontal Face ID unlock. At the moment, Face ID can only work in portrait orientation.