> Mantras like "methods should be shorter than 15 lines of code" or "classes should be small" turned out to be somewhat wrong.

Well, that's exactly what most of the books on the market teach, unfortunately. After years of programming I came to the same conclusion, too bad there aren't that many books on such topics such like cognitive load.

As I mentioned in the article - try "A Philosophy of Software Design". That kinda fundamental. It introduces such terms as "Complexity" and its types.