This is a wonderful thread, full of comparisons and deep theoretical knowledge. Awesome.
.02 from another perspective: years ago someone came up with a rule of thumb for film enlargement - that 10X was an approximate upper limit to maintain a smooth tonality in the film:print process. That's not absolute, but it seemed to make sense. That means 35 mm ix limited to about 8x10 (maybe a bit larger), MF to 22" square, 4x5 to 40" min, etc. Again, this is not an absolute rule, but it seems to be confirmed by viewing: that something happens in a film print when it gets larger than this - some of the "tonalities" are lost, or at least worked harder, and some richness seems to fall to the side. Of course, anything less than the 10X (say 5X) is really sweet.
For digital, this is harder. There is probably some way to interpolate from MB to print size, but there are so many variables, including printer quality, sharpening algorithms, and even which generation sensor was used that affect this. All this is apart from MB. I suspect we each develop a sweet spot for our own equipment and work flow.
In film, I used to like printing MF film, scanned, to 16" square - fit the paper, fit the printer, file size would be reasonable, and the images very taut. Changing to a full digital process affects all aspects of that workflow. And thus most of us probably have to find a new rationale for what works best. I'd offer the suggestion that its not any one criteria, but rather all of them put together, assembled intuitively, and after trial and error, the photographer (artist) finds the best combination for his/her needs. This varies by subject matter, marketplace, weight of the gear, what gear is in the closet, etc.
In short, there are a lot of variables in this mix.
Geoff
