We're talking here about the width of the phone itself, not just the screen. We're talking about width here, the X axis in portrait orientation. If they release a foldable phone, this will refer to the thinnest width when a screen is available that's 'fully featured' in the current (2025) sense of a screen. (So if there's a screen on the outside that's capable of running normal apps, and a different screen on the inside, this will measure the width when the outer screen is available. If there's a very thin outer screen, but it only shows notifications or something like that, it doesn't count.