It's commonly understood that the length of a line of text has a of consequence on how readable a body of text is. This is usually expressed in terms of characters, assuming some kind of average character mix. In this units, good line lengths are between 30 and 70 characters long.
Another measure is the "alphabet", which is a list of the characters in the alphabet. This assumes that the text you are reading is going to have an average mix of characters, and the optimum length for this measure is an alphabet and a half.
To help set your browser, here are a number of lines at those lengths, starting with an alphabet. The text for the average characters is a nonsense paragarph commonly used to demonstrate a font, slightly tweaked to make the line endings come out right. The line lengths for these are 30, 35, 40, 45, 50, 55, 60, 65 and 70 characters.
abcdefghijklmnopqrstuvwxyzabcdefghijklm
Lorem ipsum dolor sit amenten,
consectetaur adipisicing elit, sedd
eiusmod tempor incididunt ute labore et
dolore magna aliqua. Ut enim ad minim veniam
quis nostrud exercitation ullamco laboris nisi ut
aliquip ex ea commod consequat. Duis aute irure dolore
in reprehendererit in voluptate velit esse cillum dolore eu
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non pro
nident sunt in culpa qui officia deserunt mollit animn id est laborum
From looking at these, two things are obvious:
Well, those are the guidelines. There's a nice selection of character width lines to play with, so that you can easily set a browser to the font/width combination that's most comfortable for you to read.