If content is engaging, people will read it.
That should be enough. You can fine tune your content's copy as much as you want, but you'll need to communicate something, and depending on what you want to communicate, lengths of content blocks will vary.
Think about this: an e-commerce site will probably have short blocks of content, because they'll want you to focus on the CTA. On the other side, a news site could span articles through many many scrolls, even different pages
So, the first thing to know is how to deal with these content blocks, and once you define the layout and structure, think of typography.
Paul Olyslager has a very interesting article about this: The Optimal Text Layout is More Than Line Length . I'm borrowing part of his conclusions below:
The optimal text layout is difficult to define due to the relationship
between its variables. For example, long line lengths are said to need
more interlinear spacing to ensure that the eyes locate the next line
down accurately when executing a return sweep towards the end of the
There are, however, some guidelines to help you out:
- 12 point size font as an absolute minimum,
- don’t condense letter spacing,
- when changing line length, change the leading accordingly,
- Remember: optimal is not necessarily fastest reading line length but is the size preferred by the users,
- think about the impact on line length and leading when changing font size,
- keep the text layout clean and uncluttered and use enough white space.
Whether you know it or not, design has countless scientific formulas, and typography is probably the aspect of design where formulas are more useful: they were needed since Guttenberg, they are needed in nowaday's most complex application. If you want to know the formulas for layouts and typography, take a read to Secret Symphony: The Ultimate Guide to Readable Web Typography, which deals with Golden Ratio concept and seamlessly apply to most designs (you can test it if in doubt).
If you're looking for research, I recommend the papers Effect of Character Spacing on Text Legibility, The Effect of Letter Spacing on Reading Speed in Central and Peripheral Vision and Eye movements, the perceptual span, and reading speed (some parts aren't specific to your issue, but useful as a whole)
If you read the above cited documents and study the formulas and research on the subject, you'll quickly find different strategies to apply to your content in a consistent way. However, as in everything UX, you'll need to test and research your specific scenario