To some extent, bigger is better. But more importantly, it’s the proportions that harmonize the content. If your body’s typography is set at 24px, then make sure that the rest of the site is consistent with it. There are no hard rules to this, but the line-height should be from 1.2x to 1.4x the font size. Set the sizes, thinness and color variations based on priorities. A lot of it comes from gut feeling and trained eyes.