The width of a paragraph changes its rhythm. A very long line asks the eye to travel farther and find its way back; a very short line interrupts phrases repeatedly. A comfortable line length gives phrases room while helping the eye find the next line.
Give the paragraph a comfortable line length.
A short headline can span a wide composition; a paragraph has a different job. Keeping paragraphs to a comfortable line length helps the reader find the next line, while an occasional wider figure provides a useful pause between ideas. The illustration uses real words in a wide line, a cramped column, and a more comfortable line length. Readers can compare how the words wrap onto the next line. A label saying Readable would add little if the reader could not see how the words actually sit.
Read a complete passage at narrow and wide sizes. Look for cramped words, excessive line lengths, and abrupt changes in rhythm. Choose typography that helps readers follow the explanation and pause to consider a difficult sentence. There is no single width that makes every typeface and passage work. Dense explanation, brief instructions, and a large headline need different treatment. Judge a complete paragraph and the transition to the next one; a pleasing first line is not enough to establish the rhythm of an article.
Put it into practice.
Read a real passage in the actual typeface at narrow and wide sizes. Adjust the line length, type size, and line spacing together. Let a diagram use more width when that helps explain it, while keeping the surrounding prose comfortable to follow.
Does the layout help the reader stay with the thought?