Most of the time I write in Scrivener Composition Mode, or full screen as it used to be known before OS X came along with its own option of that name. In Composition Mode you see nothing but words. But there are different ways to see those words. The key variable is… line length. How many characters do you see horizontally across the white space of the full screen page.
You vary the line length using this control.
Slide it to the left for narrower, the right for wider. You might think that the wider the better, because that way you get to see more characters. But no. The human mind doesn’t work like that. A line length that’s too long — on screen or the printed page — becomes less readable. And since reading is important while writing then picking the right line length is important too.
Here’s a fascinating article on line length and readability.
It suggests you keep the line length within the range of 50-75 characters. Anything outside those margins will be too short or too long to be easily read. Note: Scrivener adjusts paper width regardless of the font. So the number of characters will depend on the size of the font you use. The best thing to do is play with that Scrivener control above and work out what feels best for you. Get it wrong and you won’t be reading your work as thoroughly as it should.
For me it comes in around the seventy character mark like this.