Web pages should fix their width to a size that the reader is used to; in printed publications, this is usually between 60 and 75 characters. Here we compare the paragraph widths of some popular media websites.

This chart comes from analysis I did in January 2002; I would like to update it soon to reflect websites today. The shaded yellow area covers one standard-deviation, or roughly 2/3 of the sampled sites.

Otherwise, if you leave your web stylesheet to automatically size the text widths, the text may run longer than the reader is used to. Or, picking a size too small (akin to newspaper or magazine columns) may not be beneficial at all, and could result in articles being too long.