Phones should be measured by the area of their front face, which includes the bezels, but ignores thickness . For example, the iPhone 6s is 14 inch², while the Nexus 5x is 17 inch². So, if you want a small phone, you know which one to pick, and how much of a difference there is between the two.
Why not use the diagonal of the screen, as is conventionally done? Two reasons. One, it doesn’t account for the bezels, which can take up a significant amount of space. For single-handed use, what matters is the size of the entire phone, not just the screen. You’re holding the phone, not the screen.
Not including the bezel sometimes leads you to the wrong conclusion. If I were to ask you which phone is bigger, the 5-inch Nexus 5 or the 4.7-inch iPhone 6s, it seems obvious that the former is bigger. But the iPhone has bigger bezels. When you compare the area of the phone, you’ll find that both are 14.something inch². See how ignoring the bezels leads to the wrong conclusion?
Second, diagonals are misleading because they understate the extent of the difference. If you are comparing a phone with a 4-inch screen and a 5-inch screen, the numbers indicate that the latter is slightly bigger, namely 25% bigger. But since area is proportional to the square of the diagonal , the actual difference is 50%. There’s a huge difference in size between a 4- and a 5- inch phone, not a small one.
So, phones should be measured by the area of their front face, in square inches . Whether you prefer a small or a big phone, this measurement will help you to compare and pick the ones that work for you. Reviews should stop quoting the diagonal size.
 Why ignore thickness? Because thickness doesn’t impact single-handed use as much as area. When you want to use your phone with one hand, the hand must stretch to all four corners of the screen. The thickness of the phone isn’t as important. So area is a more useful measurement than volume.
 Assuming the same aspect ratio, which is the case for phones, most of which use 16:9.
 While this metric does a great job at capturing how usable a phone is in one hand, it doesn’t capture whether the screen is cramped or roomy. If you’re interested in that, you can use a secondary measurement, which is the size of the screen in square inches. Again, whether you’re measuring the entire front face of the phone or just the screen, the measurement should be of area and not of diagonal.