Can i automatically change font size to get desired (66) "Characters per line" value?
I do not want to change magins size or textwidth, only font size.
Alternatively i'm looking for a macro which display number of characters on margin. For debugging/hand tunin purposes.
While the 66-character/per line is a common ideal setting, you cannot lock it to that, as the characters used have varying widths, especially when considering various different languages. A different approach is to measure the width of a lowercase alphabet for a given language, abcdefghijklmnopqrstuvwxyz in English, and set the textwidth to about 2.5 times this.
The information is paraphrased for The Elements of Typographic Style by Robert Bringhurst.
In the code below, edit the alphabet to language you are using.
\documentclass[english]{article}
\usepackage{geometry}
\usepackage{babel}
\usepackage{calc}
\usepackage{showframe}
\usepackage{blindtext}
\newcommand{\alphabet}{%
abcdefghijklmnopqrstuvwxyz%
}
\newlength{\textW}
\setlength{\textW}{\widthof{\alphabet}* \real{2.5}}
\geometry{textwidth=\textW,}
\begin{document}
\Blindtext
\end{document}
