Max resolution film size for large format.

[DREW WILEY]

There are photographers who deliberately used diffraction esthetically, for a softer-edged feel. For example, Meyerowitz routinely stopped down to f/90 in many of his shots. The effect was evident not only in 30X40 enlargements, but even subtly in 8x10 contact prints. His lens choices included a 10 inch Commercial Ektar and later a 250/6.7 Fuji W, I believe, which are both very sharp in normal usage - so this had nothing to do with a "soft focus lens" look; it was rather about nuance.

We should all feel free to make up our own rules, based on our own objectives and expectation. Perceived sharpness or deliberate modification thereof is just another tool in a potentially very large toolbox.

 

[Dan Fromm]

Back then lenses were uncoated, so you needed to stop down more than today.

Stuff and nonsense.

 

[Harry Callahan]

Uh-huh.

 

[Dan Fromm]

Uh-huh.

OK. Please explain why uncoated lenses need to be stopped well down. Be aware that I have several pre-WW I Tessars that shoot well near wide open. They are not coated.

 

[Harry Callahan]

Ok. If a lens is uncoated it does produce more flare than a coated lens. If you want to avoid low contrast you need to stop down the uncoated lens more than the coated lens. The OP was asking about enlarging negatives bigger than 4x5 - to (very) big prints. The bigger you enlarge a negative of a certain format, the less contrasty the print will be - and the more it matters that the enlarging lens does produce good contrast, resp. low flare.
In addition to that you cannot put as much uncoated elements into a lens than coated ones. If the elements are uncoated you can put three, maybe four groups into a lens but not more - because otherwise contrast would get too low. That`s why the Tessar lens has been invented as it has four elements, but in three groups - producing more sharpness than a triplet but about the same flare as a triplet. A modern, coated, six element lens should outperform a Tessar, even if the Tessar already is coated.
This means, an uncoated enlarging lens cannot produce as much sharpness at the same f-stop than a modern lens, so you again have to stop it down more.
Then, a hundret years ago, (most?) lenses weren`t corrected for curvature of field, so if you set the aperture to just that f-stop where the lens no longer has too weak contrast and too low sharpness - you can set sharpness on the middle of your print, but at the edges the print will be less sharp because plane of sharpness is bend.
Even with a modern lens, corrected for curvature of field plane of sharpness ain`t really flat - but less bend than on uncorrected lenses. So, again, and for the third reason you have to stop down the uncoated lens more than a modern, coated six element lens to also compensate for that.
Then you may end up at an f-stop which indeed could reduce sharpness of your print due to diffraction.