Proper spacing makes your readers feel good. Here’s 5 recommended font combinations from Google Web Fonts that have good spacing for reading long blocks of content.
Medium — 75 characters
37Signals: Signal vs. Noise — 76 characters
Zen Habits — 78 characters
Medium has one of my favorite reading environments online and they use a 22px font size. Several of my other favorite websites have adopted a font size over 20px for their content: