...resize your browser window? The text is as wide as you want it to be.
On the other hand, I absolutely hate it when I want wider or narrower lines, and resizing the window either causes useless whitespace or a scrollbar to appear.
I have lots of tabs open, and every other site I use chooses a legible width.
You think I'm going to resize my browser window narrower every time I switch to HN, and resize it wider every time I switch to a different tab?
Sites are designed. Legibility is part of design. Appropriate characters per line is part of legibility. Full stop.
Not disagreeing with your premise BTW.
Or you could just put it in a separate window of the desired width; maybe then you'd even have enough space on your huge monitor to see several sites all at once!
As I wrote the post above, I had about a dozen different windows open, all of varying sizes. Monitors have gotten much bigger, yet the users seem to have gotten worse at making good use of that space.
(Nor do I have a huge monitor, I'm on a laptop...)