[font color=\'#000000\']Thanks to Lin for the point about the importance of content; one example is that fine printed text seems to need higher resolution than most photographs.
But to a more basic point: the question of how big a print can be made with some number of pixels needs the viewing distance to be specified. One way around this is to talk about the "apparent size" of the image, meaning the angular size, or field of view.
A common standard of normal field of view is about 50ยบ, which means a print about as long on the diagonal as the distance from print to viewer. It seems that for this, current 5MP and 6MP cameras do very well. So as long as you view prints in this "normal" fashion, print size is not itself important: 6MP can probably give a 16"x20" that looks quite good if viewed "normally", from about two feet away.
So in these terms, what 8MP, and now up to 22MP, gives is the ability to view images covering a wider than "normal" viewing angle. (Or which would have been wider except that you cropped severely).
8mp allows the linear size to be about 15% greater: you can view that 16"x20" from 20" away instead of two feet. This does not seem much of a difference to me, but if it comes at little extra cost, I suppose I will take it.
Kodak's 3000x4500pixels (13.5MP) maybe lets you view a 16"x20" from as close as one 16", or a 30"x40" from a comfortable "gallery viewing" distance of about 30". Immersing oneself in a big image like that can be interesting with the right subject matter, but you have to pan around to see the whole image, and my gut feeling is that aesthetically, this is mostly of interest when the subject is itself a very wide angle scene, and you want to experience it the same way.
My own experience is that even with huge, sharp, detailed prints from large format, the last example is my extreme for field of view: viewing distance equal to the short edge length of the print. Whenever I go closer than that, I am just judging technical quality and have lost aesthetic contact with the photograph. This sets my personal maximum useful field of view, and it seems to be covered by about 12-14MP, and so I am tempted to think that for most purposes, the high end is more or less there now. (With primes, there can perhaps be more need for cropping, so maybe 22MP has its place!)[/font]
