Some rumors are flying about iPhone screen size today. Usually I ignore that sort of thing, especially since I agree with John Gruber that any sort of change to the iPhone screen size is highly unlikely for practical app development reasons. But today it suddenly occurred to me that Apple does have some flexibility built in here, flexibility I’d noticed a few years back when the first iPhone was released.

What I noticed back then was that though the iPhone has a 3x2 display (not quite the traditional 4x3), the dimensions of the phone itself could easily accommodate a 16x9 display. I thought Apple had probably been pretty deliberate about that.

The image above is the technical drawing of the current iPhone 4S. What if the rumors today are off by just a little. What if, instead of chasing Samsung’s absurdly large 4.6 inch diagonal display, Apple were to build a 4 inch or even 4.4 inch diagonal display. Would that work?

In this drawing I’ve just stretched the iPhone screen out to 16x9 dimensions. This actually fits very well. The home button would only have to shrink by about 1mm to accommodate the new screen. Of course, I’m sure the hardware engineering issues would be quite a bit more challenging than this drawing implies. However, the software challenge would not be so daunting. Legacy apps could run centered on the screen with black “letterbox” bars on the top and bottom (or sides, if rotated). Graphics would not need to scale to odd fractions. The screen would have a 1138x640 pixel display. Apple has shepherded developers through much more difficult transitions than this screen represents.

In this drawing I imagine the same 1138x640 pixel display enlarged just a bit to completely fill out the same handset size. This would create a 4.4in diagonal screen at 16x9 dimensions and greatly increase the hardware challenges. The home button would probably need a new shape and the speakers would have to be rearranged, for example. But note that this screen drop to a 296ppi resolution, well below the “retina display” mark Apple has now promised. I think this scenario is highly unlikely.

In fact, I still agree with Gruber that the most likely scenario is that Apple does not change the screen size at all. But if it does, I bet it goes to a 4 inch diagonal 16x9 display on (roughly) the same size handset, not the bloated 4.6 inch route.