It's okay talking about which diagonal screen size is ideal and how it compares to the big Android phones out there, but there's one big difference; the aspect ratio.
Apple have settled on 3:2 for all their iDevices. Everyone else has settled on 'widescreen' 16:9.
The vertical screen dimension is the important one because that defines the width of the phone overall. With any given vertical screen dimension, an iPhone's diagonal dimension will always be smaller. To increase an iPhone screen to a diagonal size of a top end Android phone for instance, would make it comparatively much more unwieldy.
A few screen size examples. What would be the ideal for a new iPhone?:
- 2.95 x 1.96 = 3.54" (current iPhone)
- 3.17 x 2.11 = 3.8"
- 3.33 x 2.22 = 4"
- 3.58 x 2.39 = 4.3"
- 3.87 x 2.58 = 4.65"
And for comparison with Samsung offerings with 16:9 screens:
- 3.75 x 2.11 = 4.3" (Galaxy S2)
- 4.05 x 2.28 = 4.65" (Galaxy Nexus)
- 4.62 x 2.60 = 5.3" (Galaxy Note)
Of course this all assumes Apple are intent on sticking with 3:2. But if suddenly they started doing 16:9, that would introduce an element of fragmentation that currently they don't have to deal with. Looks like they may have shackled themselves to 3:2 whether they like it or not.