Copyfitting is the process of deciding which typeface, point size, leading (line spacing), and line length are necessary to fit a certain amount of text into a given space. With careful planning, these choices need only be made once, at the beginning of a job.
It is important to understand how each of these choices affects readability. In general, text becomes less readable as the typeface becomes too condensed or too extended, point size becomes smaller, leading becomes tighter, or line length becomes longer.￼
If you find yourself forced to use 6-point Helvetica* Compressed Ultra with 6-point leading, it is time to start editing!