German-born designer and typographer Erik Spiekermann shares his ideas about content consumption as it relates to screen size:
There are physical limitations as to certain size. It’s nice to read 10 words a line, 50 to 60 characters. This is science. This is not me. This is something that we like, the way our eyes move in little segments. There are physical limitations to our eyes: the curvature of our eyeballs, the space we have in front of us, the distance from the eyes. That’s human, and no machine can ever change that.
There’s a certain size that looks good to us. There’s a certain contrast. Total black and total white is horrible. That’s why books are nice, because they’re not totally black or totally white. We like a little softer.
When you’re reading on your smartphone I think 10 words might be a little too many. I’d say it should be about 8 or 9 on a 4.5-inch-ish smartphone. For older folks with poorer vision the fonts will need to be even larger.
Optimal size depends on our vision. Optimal size also depends on screen size, not because the science of vision changes, but because our preferences and expectations change based on screen size. We hold our smartphones closer to us and expect things to be a bit more succinct. On a 30-inch monitor — with enough pixels (~2560×1600 or more) — we expect the visual experience of reading an electronic version of a magazine to be identical to reading the real one. A 6-inch E Ink grayscale Kindle? We expect something different yet.