The goal should be to keep each block of text compact enough to be scanned without undue eye movement and “open” enough so that copy does not feel dense or cramped.
A good rule of thumb on eye movement is that there should be no difficulty finding the next line when you’ve reached the end of the line you’re reading. Line spacing will factor in here, as will the size of the type itself, but rarely will you find a font where line heights are not well matched to the type size, so the width of your text block is usually the factor that requires adjustment to get this right.
With wider displays and a more horizontal layout to most desktop computers, this has gotten more complicated. Designers want to use the additional available space, and usability demands that we don’t have text blocks that run across the full width of these wide screens. Add in the variety of mobile screen sizes and it becomes apparently very quickly how important it is for your design team to define how text should lay out on all devices.
None of this is terribly complicated, but that doesn’t mean it’s easy to get it right. Usability in general can get lost in the shuffle, particularly for web development projects on limited budgets. And readability is even easier to overlook — type just isn’t as sexy for most designers as, well, design.
Stick to your guns, though, and you’ll be rewarded with a site that is more likely to engage your audience, and that’s always the bottom line. Regardless of what any expert might say about usability, design, or anything else, if your audience is creating engagement and generating business, it’s doing its job.