In addition to letter spacing and line height, the number of characters per line is also important because a long line of text produces fatigue and a short line of text can be distracting. 75 characters per line, including spaces, is usually the maximum for a block of text with 5o-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 text instead of moving from left to right because the quick movements side to side fatigue the eyes.
Since line length is so important as a fatigue factor, web pages should be set to a fixed width rather than being liquid, allowed to contract and expand as readers change the size of their web browser windows. With liquid layouts, readers can become fatigued and stop reading for reasons that have nothing to do with your design.
Designing for readability thus requires that the size of type should also be in relation to the width of the text. A narrow text will require smaller type in order to maintain a line width that is conducive to being read.
Also see Golden Rule Typography Calculator for a tool to calculate these relationships.
