Well, yeah, I realize there are a lot of parameters to consider. What I meant was, and perhaps this is also difficult to figure out, but imagine you take the same picture, same lens, same f-stop, with two different cameras, say, a D4 and a d800, and you would make first a print in each camera's native format, at 300 dpi - or higher, if you like that idea better. What you should get, provided you got the exposure right, is an ideally sharp print, no compromises. From which size onwards for each camera would you think it likely you have to make any compromises which you can't cover any more by whatever crafty interpolation?
Lukas
Trying to be a keeper of the light
http://www.flickr.com/photos/lukaswerth/
also see my Nikonians gallery:
https://images.nikonians.org/galleries/member.php/uid/444897