Currently the iphone screen is just big enough that I can hold it with one hand and reach all four corners with my thumb. This reminds me a bit of the argument

Currently the iphone screen is just big enough that I can hold it with one hand and reach all four corners with my thumb. This reminds me a bit of the argument that 90 feet is the optimal size for a baseball infield, as evidenced by the number of close plays at first base after a ground ball to an infielder. In actuality, though, the high frequency of close plays is the result of how the infielders choose to position themselves. You can reach all four corners of the screen with your thumb not because of the size of the phone, but because of how you choose to hold it. Change the size of the phone, and you'll naturally change your grip.