A few people have expressed their disdain that all three of this year's new models are larger than the 6/7/8/SE. With Apple choosing to kill the SE instead of spec bumping it, I would assume we won't see anything that small again, nor do I think Apple will release any updated models going forward with the traditional home button design. If we assume that there is enough demand for an iPhone smaller than those released this year, and Apple did choose to offer a smaller iPhone with the notch and Face ID, I can only think of two possibilities.
Option A
5.5" LCD, 1624x750, 326ppi
The iPhone XR takes the iPhone XS Max resolution, dials it down to 2x instead of 3x scaling, and maintains the traditional 326ppi so that UI elements are rendered slightly smaller than the Max, and thus the display is slightly smaller. This option does the same thing with the regular XS resolution. This display would be exactly the same width as the 6/7/8 display, just taller. Assuming a bezel width the same as the XR, this results in a phone that is very slightly wider than an 8 and very slightly shorter (almost imperceptible so pretty much the same footprint).
Option B
5.0" OLED, 2082x960, 458ppi
This display takes the 5/5s/SE resolution, bumps it up to 3x scaling, and lengthens to the new aspect ratio. UI elements are rendered slightly larger for parity with the other OLED models. Assuming a bezel width the same as the XS, this results in a phone slightly larger than an SE, but still nearer to an SE than an 8.
I think if Apple does feel the need to reintroduce a smaller iPhone going forward, Option A is more likely. As we know from the XR, LCD displays allow for more affordable price points. Option B requires developers to support another new resolution, whereas Option A does not. Option B is also likely too small to have broad appeal. With the trend towards larger displays, Apple can probably get away with not offering anything smaller than the 7/8 footprint considering there are very few phones that compact with flagship or near-flagship features.
To give you an example of how Option A could fit into the lineup, this is how it could've worked this year:
XR (5.5" LCD) - $649
XR Max (6.1" LCD) - $749
XS (5.8" OLED) - $999
XS Max (6.5" OLED) - $1099
The standard XR and XS display the same amount of content on the screen, and the same is true for the XR Max and XS Max. The proportional size difference between the XR and XR Max would be approximately equal to that between the XS and XS Max.
Option A
5.5" LCD, 1624x750, 326ppi
The iPhone XR takes the iPhone XS Max resolution, dials it down to 2x instead of 3x scaling, and maintains the traditional 326ppi so that UI elements are rendered slightly smaller than the Max, and thus the display is slightly smaller. This option does the same thing with the regular XS resolution. This display would be exactly the same width as the 6/7/8 display, just taller. Assuming a bezel width the same as the XR, this results in a phone that is very slightly wider than an 8 and very slightly shorter (almost imperceptible so pretty much the same footprint).
Option B
5.0" OLED, 2082x960, 458ppi
This display takes the 5/5s/SE resolution, bumps it up to 3x scaling, and lengthens to the new aspect ratio. UI elements are rendered slightly larger for parity with the other OLED models. Assuming a bezel width the same as the XS, this results in a phone slightly larger than an SE, but still nearer to an SE than an 8.
I think if Apple does feel the need to reintroduce a smaller iPhone going forward, Option A is more likely. As we know from the XR, LCD displays allow for more affordable price points. Option B requires developers to support another new resolution, whereas Option A does not. Option B is also likely too small to have broad appeal. With the trend towards larger displays, Apple can probably get away with not offering anything smaller than the 7/8 footprint considering there are very few phones that compact with flagship or near-flagship features.
To give you an example of how Option A could fit into the lineup, this is how it could've worked this year:
XR (5.5" LCD) - $649
XR Max (6.1" LCD) - $749
XS (5.8" OLED) - $999
XS Max (6.5" OLED) - $1099
The standard XR and XS display the same amount of content on the screen, and the same is true for the XR Max and XS Max. The proportional size difference between the XR and XR Max would be approximately equal to that between the XS and XS Max.