(Disclosure: I work for Google, but not on Android or anything mobile-related.)
Apple announced large iPhones, going all the way up to 5.5-inch iPhones, which was until recently phablet territory. This rekindled the discussion about what size of phone works the best, and reminded me of my own progression with larger phones.
My first smartphone was the HTC Dream, at 3.2 inches. I then switched to the 3.5-inch iPhone 3GS for a couple of years, then slowly moving up to the 4-inch Nexus S, and then to the 4.6-inch Galaxy Nexus .
At first, I missed the convenience and single-handed use of the 3.5-inch screen. I remember standing in a shop, with a lot of heavy stuff in one hand and, with my other hand, taking my iPhone out of my pocket, unlocking it, opening my shopping list, and verifying that I bought everything I wanted to. I could reach the top of the screen — the back button — in single-handed use, to navigate around.
With bigger phones, I could not do this. Single-handed use was mostly out — either downright infeasible, or inconvenient enough that it was not worth the hassle. At first, I missed being able to use the phone with one hand, but over time I got used to the larger screen size. And to being able to see a lot more information at once on screen. Which was convenient, and let me do things like check in to flights, which were too much trouble to do on a 3.5-inch screen. In other words, it was not just about efficiency, but about being able to do things I couldn’t do on a smaller screen. Eventually I got used to the larger screen, and it became second nature.
After years of using these larger-screened phones, I remember looking at a friend’s 4-inch iPhone 5 and thinking, “That’s such a tiny screen. How can you get anything done on it? What’s the point of having a smartphone if you can’t use it to get things done?!”
After a couple of years of using the 4.6-inch Galaxy Nexus, last year I bought the iPhone 5s, returning to the small screens camp. I bought this phone because I decided that photography is for me the most important aspect of a phone. I then understood the other side of the coin — a smaller phone was easier to use in short bursts, like taking my phone out of my pocket, taking a few photos, and putting it back. Or taking it out, sending out a quick mail or SMS, and putting it back. Most of my usage was in short bursts , and for that the smaller screen actually worked better.
If, on the other hand, you tend to often use your phone in longer sessions, like 30 minutes each, then a large-screen phone would work better for you .
The argument went that we anyway have tablets, which are far better at what they do than phones, so buy a gigantic phone, and end up with a compromised device that’s neither a great phone or nor a great tablet? This argument ignores the hundreds of millions of people who have no other Internet-connected device than a smartphone. Even if you have a tablet, it’s not always with you, it doesn’t fit in your pockets, it doesn’t have cellular connectivity , it may not have the same apps installed and available in the same locations on your home screens, and signed in to the same accounts. Even if it does, sync is nowhere as seamless as just having one device.
Besides, the tablet market is cooling, as Marco Arment points out — tablets are being squeezed by bigger phones on one side, and by ever-better laptops on the other. For more and more people, a tablet is a poor compromise between a phone and a laptop.
In other words, if the argument is that a phablet is a compromise between being a great phone and a great tablet, the counter-argument is that having two devices is a compromise by itself.
There’s also a difference in the position phones occupy in our lives. When they first become popular, smartphones were looked at as secondary devices — great devices to use when you’re not in front of your laptop. But now, they are primary devices for more and more people. Even for me, who as a power user earlier thought of my laptop as my primary device, and smartphones as a device to use when I wasn’t in front of my laptop. But now my view has changed. I no longer think of my phone as a secondary device, or as a device to use only when I’m out. It’s a peer to my laptop in its importance and in that all my data and apps are there, and it’s always available. It’s every bit as important and central to my life as a laptop is .
For a primary device, screen size is important. You can’t do the things you want to on a 4-inch screen, either efficiently or at all. Reading desktop web pages is such a pain, for example, that I often don’t bother. I just save it to Instapaper or email it to myself and move on. Efficiency is one thing, but not being able to do what your want because you’re using too small a screen is a bigger problem, because it’s a difference of kind and not just extent.
I also notice that I hardly use my iPhone when at home, switching instead to a tablet or my laptop. This is a strong argument that 4-inch screens are too small. I expect my phone to work everywhere, in the house and outside.
A 5.5-inch screen is not somewhat bigger than a 4-inch screen; it’s much bigger. It has twice the screen area, which is the factor that matters. We need to stop measuring screen sizes by the diagonal, because it’s misleading — it falsely implies that a 8-inch screen is twice as big as a 4-inch screen. Actually it’s 16 times as big (the area is proportional to the square of the diagonal). It’s the 5.5-inch screen that’s twice as big as the 4-inch one .
I also hope that a 5.5-inch screen will let work better for photography, because I can see what I’m about to photograph or what I’ve just photographed. With a 4-inch or smaller screen, whether on my phone or my high-end camera, I often take photos that look good on the small screen, but lose their appeal when viewed at a reasonable size, such as on a laptop screen. A good camera is one that lets me take better photos, by making me aware of how they’ll turn out.
In any creative field, you try various things, and then see how well, or not, they worked. Shortening the feedback loop lets you make quicker progress and develop your skills faster. With a large screen, I hope to have a negative feedback cycle, where I know whether a photo will come out good even before I take it. What more could I ask for?
Since there are so many factors that affect what kind of phone works well for you, one conclusion is that one size doesn’t fit all. The smartphone market is big enough, with a billion of users, that there’s room for a healthy variety of devices rather than just one or two .
 It’s funny that screen size keeps increasing gradually, by only half an inch or so each time. I guess people are willing to tolerate only gradual change. Make a phone too big too soon, and they’ll say it’s too big, but they’ll end up using it after a couple of years.
 Then again, maybe most of my usage is in short bursts because a 4-inch screen doesn’t lend itself to much more. Reading desktop web pages is such a pain, for example, that I often don’t bother. I just save it to Instapaper or email it to myself and move on. Maybe if I had a 5.5-inch screen, I’d find myself using the phone in longer sessions. This screen size issue is such a hard thing to reason about.
 Assuming, of course, you have large enough hands and pockets, which excludes many women.
 Would you like yet another monthly bill to pay, in addition to paying significantly more for a tablet with cellular data? It’s not worth the hassle.
 The conclusion to this line of thought is a phone to which you can connect a monitor and keyboard to make it a laptop. Most of the things I do on my laptop don’t require a powerful quad-core Intel Core CPU or 8GB RAM. Or the speed of USB 3, Thunderbolt or Ethernet. With more and more things moving to the cloud, and apps being just interfaces to your data on the cloud, to which the device can offload the heavy lifting.
I remember using Windows 3.1 on a 386 running at 25Mhz with 8MB RAM. There’s no reason why a phone with a multi-core gigahertz CPU with a few GB of RAM won’t be able to serve as your PC?
Of course, there will always be a core set of users who need lot of local compute — programmers, Photoshop users, video editors, high-end gamers, etc. But for most people, a phone is powerful enough to serve as a PC. This is an 80/20 argument.
 The other reason we need to stop using the diagonal measurement is that it leads us to the wrong conclusions when comparing screens of different aspect ratios. A 14-inch screen can be bigger than a 15-inch one. And it is, if the 14-inch screen is 16:9, and the 15-inch screen is 21:9. This makes it hard to compare screen sizes and pick the one that works best for you.
This also causes other negative effects like tempting manufacturers to make widescreen displays because they can quote a bigger number in their marketing. Manufacturers should choose whatever aspect ratio works best for their users, rather than playing these misleading games. Which is another reason to switch from a diagonal measurement, in inches, to an area measurement, in square inches.
 I’m not saying that there’s a need in the market for a 5-inch screen and a 5.1 and a 5.2. But there’s certainly room for a 5 and a 5.5 or a 6.
 As some other examples of niches: Phones that are thicker but last for multiple days, or a full day no matter how heavily you use it . Phones that have an inbuilt credit card reader, for all the delivery people and shopkeepers.
E-ink phones for people who don’t care about photos and videos, but care primarily about reading. Maybe 98% of users wouldn’t want an e-ink phone. That’s fine — that still leaves a market of 20 million users (given the smartphone market size of 1 billion). And that’s not just 20 million users who would be willing to buy your product, but 20 million for whom an e-ink phone will work better than alternatives.
Another example is photography. Why don’t we see more phones with large 1/1.5-inch sensors, F1.4 lenses, optical image stabilization, zenon flashes, a builtin zoom (even if it’s only 3x), perhaps a builtin tripod for night photography, multiple cameras in one phone http://photo.stackexchange.com/questions/47082/why-arent-devices-with-two-cameras-more-popular and so on?