Having a paragraph extend the entire length of a page is practical, but after a few lines, readers reach the end of the line only to hunt for the beginning of the next. On the other hand, if a paragraph is too narrow, the eye must always jump back. The words will break at odd places, creating a jumpy rag, and the paragraph will appear longer than the content needs to be.
Line length, or measure, is the width of a line of text in a horizontal direction. When talking about measure, we often refer to a specific count of characters. But, the number of characters is just a guideline, as various elements will affect the line length. For example, a narrow serif typeface can give very different measure than a wide sans serif in a container of the same width, while also taking into account the font size, x-height, words that are being set and how the type is being set to be read.
When beginning, people often adjust font size when they should adjust column width. If text is not readable, you must first observe the distance from the start of a line to the end. Long lines can result in skipping or rereading the same line, and short lines can disrupt the flow of a sentence, and may even force extra hyphenation. You want your eye to comfortably travel along a line to return to the next baseline.
When comparing measurements, copy a paragraph into 3 textboxes of varying widths, without changing the typeface, font weight, font size, leading or alignment. You can then have 3 paragraphs, 1 narrow, 1 medium, and 1 wide. Read them while at their size and not while zoomed in. See if your eye loses its place, if the line looks too wide, the text feels off-center or if you feel that the punctuation of a sentence repeatedly falls at odd places in the line.
In addition, you have to take into account the space you are working in. When it comes to a wide desktop space, a line length can be set smaller to provide room on the sides of the text. But, when you are working on something more limited, such as a screen, you might not be able to fit the paragraph within the margins. A small column width can work better for a caption, since it does not have that much information in it. A longer text, on the other hand, might need to be wider to encourage comfortable reading. You want to avoid making the body of text any wider than needed to take up space, as white space could be used instead to separate from the rest of the layout.
Once you find a good width, you should adjust the leading, paragraph and character space incrementally. A wider line can be better adjusted if there is more space between lines. The space between lines will also help your eyes when making that jump to the next line after a wider measure. You should also look at the paragraph, rather than only at individual lines. A good measure will have a somewhat steady line of words (rag), without too many hyphenation marks and will allow a couple of sentences to remain together without looking like a large block of text on a single line.
When you are done, export your document and print it out or view it at its intended size. Look at one paragraph or more to see where you are having trouble with reading, either in the way you go too slowly through it or if you end up rereading the same line. Narrow or widen the column and test it again to find the right measure, one that lets the reader to easily read the text, while also fitting in the rest of your layout.