For what customers want in regards to size, it's a question that can be actually answered. It's not so much a matter of opinion, but a series of opinions.

Nokia of course has vast legacy knowledge of what size of pocketable and hold-on-your-one-hand-while-talking phone device consumers want, because of all the S30/S40/Symbian devices. It can be empirically studied, with prototypes of various sizes (weight, width, height, thickness, screen size etc.), and it has been, almost to death. And Nokia isn't alone there, I guess virtually any manufacturer have done those studies.

In the same norm it is equally possible to study preferred sizes of devices where talking to it while holding with one hand isn't a critical element but "maximum-pocketability" still is, and then to study mobile devices that are "easy to carry", i.e. not in your pockets anymore but in backpacks and purses and places like that.

I.e. this tries to be a friendly way of saying that we do know what size devices consumers want. Naturally the numbers are more like Bell curves than absolute ridges. Most people can't tell the difference within a certain size variation.

In general, the meaningful size for most consumers is the size of the device, not of the display. Then the display tries to be as large as possible, given the constraints of a particular device.

I'm personally somewhat amused by all the speculation here about 4.1" and 3.5" screens. It's all very doom and gloom whereas imho it doesn't make much difference one way or the other. I use N810:s and iPhones interchangeably, and I don't really care. Because I know, I do observe the N810 screen to be slightly larger, but the knowledge of this doesn't really comfort me one way or the other.

I hope that the next Nokia Maemo device comes out with a 3.8" screen: that way on paper nobody will be happy.