[Tex/LaTex] How to choose font size given the paper size

fontsizememoirpaper-size

How do you choose what font size to use given the paper size in use?

For example, I found somewhere that for a European standard the following is recommended:

\documentclass[a4paper,11pt]{article}
\usepackage[scale=0.7,vmarginratio={1:2},heightrounded]{geometry}

What is the typographic rule here? For example I want to print something (book, dissertation) on an A5 paper size. What should be the corresponding font size? How should I adjust the boundaries?

To be even more specific, in case it makes a difference, how this issue should be tackled when using the memoir document class?

Edit:

Thanks to @cfr's answer, I'll state a more explicit case. I use the memoir class and it will be printed on A4 page. The class is loaded as:

\documentclass[11pt,a4paper,extrafontsizes,twoside]{memoir}
\fixpdflayout

Furthermore, I load the package \usepackage{fourier} to use the Utopia face.
Finally, using the code snippet from 15 of the memoir class, I get the following output:

The length of this alphabet is 39.03668pt.

Should I do anything further?

Best Answer

The problem is not the font size nor the paper size, but the numbers of characters per line.

As noted in others answers, around the 65 ± 15 characters is considered a reasonable cut-off regardless the document type and even the font size.

When line of text are too long, your eyes will have a hard time focusing on the text. Its difficult find where the next line starts, specially in large paragraphs. Think in a landscape A0 poster with a single column of text ...

If the line
is too short
break the reader
rhythm, since
the eyes will
have to travel
back too often.
This is so
annoying that
dancing eyes
can skip
some words.

You can roughly estimate the Average number of characters in one line. If you find that this number is out of a reasonable range, your options are basically:

  1. Modify the margins. People are often obsessed with layouts that fit in mathematical ratios (golden ratio of 1.618:1, etc), geometrical rules, traditional styles, etc., but IMHO, the best way to judge a layout is your eyes, not any arbitrary type of complex algebra. Here you must consider mainly two factors:

    a. Readability. Let these margins take the document in your hands without putting your fingers on the text? Do you see the text beside the inner margins of the book in a true 2D plane or in the border of a black hole? The part of the screen around the PDF is distracting you?

    b. Appearance. The margin of a text, like a picture frame, is a ornament that highlight the content. The ornament depend of the text. A luxury book need big margins, but not a shopping list.

  2. Modify the font size. Try to stay in the 10-12pt range as far as possible, but the important point is a comfortable viewing at about one arm's length (or more for a poster). In any case must be still easily readable, i.e, even if the line is less than 50 characters, do not use a 8pt characters for readers over 40 years. People with presbyopia will curse you for it. A very big font is harder to read too.

  3. Use two or more columns. For any bigger pager size than a A4 paper is really a must. Magazines and newspapers always use mul­ti­ple-col­umn layouts for this reason, never single blocks of text with one inch margin or so. Even for articles in A4 with a 10pt, IMHO is more pleasant two columns with reasonable smaller margins than a single column with the default margins.

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