Stanley starts with the given that for readers “the optimum line length is broadly somewhere between 60 characters and 75 characters.” That apparently has to do with how much we can take in, how easily our eyes can find the start of the next line, and so on.
As it happens, we have ended up with paper sizes that were never designed or adapted for printing with 10-12 point proportionally spaced type. They were designed for handwriting (which is usually much bigger) or for typewriters. Typewriters produced 10 or 12 characters per inch: so on (say) 8.5 inch wide paper, with 1 inch margins, you had 6.5 inches of type, giving ... around 65 to 78 characters: in other words something pretty close to ideal.Thus, like non-curly quotation marks, the double hyphen in place of an em-dash (as in Stanley's posting), and the double space after a period, our “letter size” page was based on working within the limitations of a typewriter in the early 1900s.
But if you type in a standard proportionally spaced font (worse, in Times -- which is rather condensed because it was designed to be used in narrow columns) at 12 point, you will get about 90 to 100 characters in the line.
Stanley goes on to discuss various ways text designers can produce more easily read text on that standard sheet.
Note, however, that at the bottom of that page commenter Alfe says that the limit of 60-75 characters is itself a cultural norm, not a scientifically validated fact.