Now that it’s official that Apple will be holding an event on September 9th, speculation has turned once again to exactly what size the various iPhones Apple might launch will be. Much of the debate so far has focused on whether there will be one or two new sizes: 4.7 inches seems to be a universal certainty, but there’s some debate over whether or not we’ll see a 5.5-inch phablet-style iPhone. My Techpinions colleague Ben Bajarin has a great, skeptical, take on the prospects for such a device.

I’m not quite as skeptical as Ben, but I’m also not totally gung-ho about the phablet category. It’s particularly dangerous to extrapolate demand for iPhones at different screen sizes from Android purchasing behavior, for two main reasons:

• iPhone and Android users behave very differently, in a whole variety of ways: Android users spend less on devices and apps, spend less time in apps, download fewer apps, are more likely to live in emerging markets and in Asia, and so on and so forth. They’re simply very different user bases, and there’s no particular reason to believe they’ll behave the same way when it comes to screen sizes when their behavior is so different in every other way.
• Secondly, and perhaps more importantly, it’s very easy to reverse cause and effect with screen sizes in the Android world. Many people seem to assume that, because most premium Android devices are larger than 4.5 inches, that must be what people want. But the reality is that it’s almost impossible to buy a premium Android device with a screen smaller than 4.5 inches. So, the question becomes, are premium Android devices only made in sizes above 4.5 inches because that’s all anyone wants, or is that all anyone wants because that’s all that Android OEMs make? I’d argue that Android device makers have very deliberately targeted the larger size as a way to set themselves apart from the iPhone, but that doesn’t necessarily mean it was demand-driven.

For these reasons, I’m skeptical that we’ll see the same share of sales by screen size with the iPhone as we’ve seen with Android, even if Apple does release a 5.5″ phone. Apart from anything else, there are people who have very deliberately stayed with or switched to the iPhone precisely because it fits their hand better.

That raises another question few people seem to have tackled: will there still be a premium iPhone, i.e. one with the same specs as the 4.7″ and 5.5″ models, at 4 inches? In other words, will it be possible to buy the equivalent of the iPhone 5S at the same screen size in late September, or will the 4-inch screen be the lonely province of the 5C equivalent? And if that’s the case, does it mean that this screen size gets phased out altogether next year or the year after? I’d like to see the 4″ size stick around in the premium tier for at least another year, just to give customers a chance to vote with their feet. If no-one buys the 4″ device, Apple can jettison it next year. But I suspect there are people who like the 4 inch size and will find it difficult to abandon. Having said that, of course, if Apple stops making that size, where else will those customers go? The iPhone is already the only premium handset being made at that size.

The last question is how these three sizes might sell. My guess is that if all three sizes stick around, the 4.7″ model will sell best, followed by the 4″ model, and lastly the 5.5″ model. If the 4″ model doesn’t stick around, then the 4.7″ model will vastly outsell the 5.5″ model. If it’s just 4.7″ and 4″, it might be 70/30 in favor of 4.7″. Of course, a lot depends on the pricing. If it’s strictly tied to size, and each size bump triggers a big price increase, that’ll tip things significantly in favor of the smaller devices. I suspect Apple might give the larger devices more storage capacity too, as a way to bridge the gap, such that they start at 32GB instead of 16GB. Regardless, it’ll be fascinating to watch.