Obviously, care should be taken to make sure that text is not presented too small. It’s also important to remember that the age of your audience may vary, hence the quality of their eyesight. Generally, it’s good to stay around 13px or .813em at smallest. Currently, with the wider implementation of responsive websites, there’s a trend moving toward larger body copy. In RWD, it’s also important to keep in mind that different text sizes for different devices may make sense. For instance, it may make sense to increase body copy size on a mobile phone width as opposed to a desktop width.
Another common practice that hinders type readability is allowing the horizontal lines of type on a page to become too wide. This distance is referred to as measure (also sometimes line length). If a line of type is too long it’s a tedious read and a stretch for the reader’s eye to return to the left edge of the text block for the next line. It’s also intimidating to see a block of text arranged this way and some readers may not even attempt to read it.
So what’s the maximum width a text block should be? Well, it all depends on the size of the text. The larger the text, the longer the line can be (that whole proportion thing again). In my opinion, generally around 70 characters is as long as you want to be. On average, for body copy sized text, I try to stay within 45ems.
Letter spacing (also referred to as tracking) is the consistent increase or decrease in distance between the letterforms in a word or block of text. It’s not to be confused with kerning, which refers to adjusting the distance between individual characters. Letter spacing can be used to adjust the density of a block of text or an individual headline or subhead.
Obviously, letter spacing does affect the readability of text. Too much or too little and readability will be compromised. However, there are times when, in my opinion, letter spacing is needed. As you can see in the graphic below, I like to add generous letter spacing to subheads or phrases of uppercase text. I find it's easier to read uppercase text when the characters have some additional space around them. Also, depending on the typeface used, I like to increase letter spacing slightly in body copy.