When people read something in a difficult-to-read font, they transfer that sense of difficulty onto the topic they're reading about.

Norbert Schwarz, a leading fluency researcher, and his former student Hyunjin Song have found that when people read about an exercise regimen or a recipe in a less legible font, they tend to rate the exercise regimen more difficult and the recipe more complicated than if they read about them in a clearer font.

The same goes for products and purchases. Nathan Novemsky and his colleagues did a study on this called Preference Fluency in Choice. They manipulated the fluency of a product by listing its features in either an easy or hard to read font.

Easy to read fonts doubled the number of people willing to purchase the product.

Your font should never be below 14px - and since people use website more and more on mobile devices with small screens and/or HD monitors with 1080p resolution, 16px is even better.