One of the biggest changes we're introducing in the Desktop improvements project is a width limitation on the content. Currently there is no width limitation so users on wide monitors may see line lengths that are well beyond what we understand to be the recommended amount (45–75 characters per line). We understand this to cause a readability issue.
As we've dug deeper into the research we've encountered a problem: all of the research we have found thus far is studying line lengths within the 35–125 characters per line (cpl) range. Therefore it is difficult for us to prove directly through existing research that our current layout, which may result in line lengths well over 200 cpl, is problematic. I think we can safely infer that it is an issue, but it would be great if we had specific research we could cite. In other words, if there was a study comparing line lengths of 100 cpl vs 200 cpl vs 300 cpl that would be more helpful to us than the studies on more narrow ranges.
So far the studies we have read are here: https://docs.google.com/document/d/1by9828Om8mlTiAVrfGOfZqBhrnXRmfG0IMsIPiFVFR0/edit?usp=sharing
The closest thing we've found thus far is here, however unfortunately the test was never completed: https://www.gwern.net/AB-testing#max-width