Line Length
Nobody sets a line length. It gets set for you by whatever container the text landed in, and on a wide screen that container is usually the window. The result reads as hard work without looking like anything is wrong, which is why it survives review.
The cost is at the end of each line. Coming back to the start of the next one is a jump the eye makes without looking, and it lands by distance. Past about eighty characters it stops landing reliably, you re-read a line you have already read, and the paragraph starts to feel like effort.
Cap the measure at 60 to 75 characters. The unit is characters, so use ch and let the font size do the arithmetic.
The measure is the number of characters on a line, and it is the single thing that decides whether a block of text gets read or skipped. Too long and the eye loses the start of the next line on the way back. Too short and it breaks the sentence into pieces before you have finished it.
No limit. Around 110 characters on a wide screen.
The measure is the number of characters on a line, and it is the single thing that decides whether a block of text gets read or skipped. Too long and the eye loses the start of the next line on the way back. Too short and it breaks the sentence into pieces before you have finished it.
max-width: 65ch. The line ends where the eye expects it.
Why ch and not pixels
A pixel width is only correct for one font at one size. Change either and the measure changes with it, silently. The ch unit is the width of a zero in the current font, so a cap written in ch stays the same number of characters through a type change, a size change, or someone bumping the root font size in their browser.
It is an approximation, because a zero is not the average character width in most faces. It is close enough that the difference never shows up, and it is the only unit that tracks the thing you actually care about.
Where it goes
On the text, not on a wrapper. A wrapper capped at 65ch also caps the images, the code blocks and the tables inside it, and those usually want the full width. Put the limit on the paragraph and let everything else decide for itself.
Short text is exempt. A heading, a label, a caption and a button all sit well under the limit already, and capping them does nothing except add a rule someone has to read later.