The number of characters and words on each line of text, known as “measure”, can have a dramatic effect on readers’ ability to comfortably move through a column of text. If measure is too thin the flow of reading is disrupted by the constant need to jump from line to line, while a too-large measure can make it hard for the eye to find the next line while moving down the page. Both extremes hinder readability and make reading less comfortable for many users (including some who prefer longer lines for other reasons).
Prevailing typographic wisdom is that each line of text should have about 50-75 characters including spaces. This can be achieved with any potential column width or font size by balancing the two values together for the desired result. In many cases an existing design will need either a significant increase in font size or decrease in column width to hit the sweet spot.
Fluid-width (responsive) web design presents a particular challenge to maintaining a readable line length because the design has to accommodate all possible widths for any given column of text. In some cases like small phones in portrait orientation compromise is probably the only option (and a shorter line length is likely more efficient for handheld use anyway). For the rest of the spectrum a combination of maximum column width (to avoid enormous text) and careful font size management across breakpoints should help maintain a reasonable line length for visitors on any device.
Resources about line length
- Readability: the Optimal Line Length from the Baymard Institute
- Choose a comfortable measure from The Elements of Typographic Style Applied to the Web.
- Line length and readability: speed vs. user experience, users prefer thin line lengths even if they read more slowly.
- The Line Length Misconception From Viget (dissenting viewpoint, good discussion in comments)
- Ideal column width for paragraphs online, answers on the User Experience StackExchange site.