The most useful way to measure line length is by average characters per line. Measuring in inches or centimeters is less useful because the point size of the font affects the number of characters per inch. Whereas characters per line works at any point size.
Shorter lines are more comfortable to read than longer lines. As line length increases, your eye has to travel farther from the end of one line to the beginning of the next, making it harder to track your progress vertically. Aim for an average line length of 45–90 characters, including spaces. You can check line length using word count.
Alternatively, use the alphabet test to set line length. This rule of thumb is worth memorizing, as it can be used even in the absence of a word-count function. You should be able to fit between two and three alphabets on a line, like so:
= 2.31 alphabets
This is especially useful when you don’t have easy access to word counts, for instance in a web layout.
How do you adjust line length? Generally you’ll want to establish the basic length using page margins. True, you can also set the side indents of a paragraph through paragraph-level formatting, but you should reserve that for special situations (for instance, block quotations).
If you plan to use indenting to distinguish sections or hierarchies within your document, take this into account when setting up the initial line length. You want to start with long enough lines so that the indented parts also fall within the target range. Using fewer levels of indentation, and smaller indents, will help.
The major flaw in many responsive web layouts? Insufficient attention to line length. See responsive web design.