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. Average characters per line works independently of 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. You should be able to fit between two and three alphabets on your 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.
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. Navigation and images are carefully engineered to scale up and down, but the body text is allowed to reflow hither and yon. Not good. If you need the body text to fit a broad range of widths, then use adjustments to point size and page margins to keep the line length in the right range. Don’t just rely on reflow.