Smashing Magazine on Balancing Line Length And Font Size In Responsive Web Design:
65 characters (2.5 times the Roman alphabet) is often referred to as the perfect measure. Derived from this number is the ideal range that all designers should strive for: 45 to 75 characters (including spaces and punctuation) per line for print. Many web designers (including me) apply that rule directly to the web. I’ve found, however, that we can reliably broaden the range to 45 to 85 characters (including spaces and punctuation) per line for web pages.
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While there is no perfect line height, a good rule of thumb is to set it at approximately 150% of the font size.
I prefer not to go over 80 characters. Other than that I like these suggestions.