In addition to letter spacing and line height, optimum line length, the number of characters per line, affects readability and user experience. A long line of text produces fatigue and a short line of text can be distracting. Seventy-five characters per line, including spaces, is usually the maximum for a block of text with 50-60 being the optimum range. Unless a line is short for effect, such as emphasizing relationships between words as in poetry, readers begin to skip words as their eyes move down the page instead of moving from left to right, because quick side-to-side movements fatigue the eyes.
Responsive vs. Fixed Website Design
Since line length is so important as a fatigue factor, the traditional advice is that web pages should be set to a fixed width rather than being liquid, which means they are allowed to contract and expand. On a desktop computer, a fixed width remains a fixed width even when the window if much wider. With liquid layouts too narrow to read comfortably, readers can become fatigued and stop reading for reasons that have nothing to do with your content. However, since so many people are reading on so many different sized devices now, the new advice is that responsive is a plus.
Designers generally don’t like responsive page design because it prevents controlling the look of the page. It allows graphics to move around and headlines to end up on two lines with word breaks that can change comprehension. A compromise is to have two layouts, one responsive for mobile devices and tablets and one fixed for desktops. (This is almost twice the work.)
The Golden Ration Typography Calculator
A narrow text will require smaller type in order to maintain a line width that is conducive to being read. If you are designing specifically for a device with known measurements, like an iPad, you can adjust type size and line height accordingly.
Also see the Triple Threat post Golden Ratio Typography Calculator.