Five Simple Steps to Better Typography: Measure the Measure
http://www.markboulton.co.uk/journal/comments/five_simple_steps_to_better_typography/

Boulton, Mark
Mark Boulton
2007

Abstract:

There is an optimum width for a Measure and that is defined by the amount of characters are in the line. A general good rule of thumb is 2-3 alphabets in length, or 52-78 characters (including spaces). This is for legibility purposes. Keep your Measure within these guidelines and you should have no problem with legibility. Please note that this figure will vary widely with research, this is just the figure I use and it seems to work well as a generally rule of thumb.