Appearance → Customize → Layout. Two sliders, at the top of the section.
The two numbers
Content width — default 1170px. The page container: the header bar, the footer, and anything set to Full. Steps: 600, 700, 800, 900, 1000, 1080, 1170, 1240, 1320, 1400.
Narrow width — default 640px. The reading column: anything set to Narrow. Steps: 300, 400, 480, 540, 580, 640, 700, 760, 840, 920, 1000, 1100, 1200.
Both are sliders over a set of stops rather than free number boxes, because a width is judged by looking at it. Drag with the preview beside you.
|<------------- content width (1170) ------------->|
| |
| |<-- narrow width (640) -->| |
Picking a narrow width
This is the line length of your articles, and it is the single most consequential setting in the theme. Typography’s rule of thumb is 45 to 75 characters a line; below that the eye jumps too often, above it the return sweep starts losing the next line.
At the default 16px body size, 640px lands around 70 characters — the top of the comfortable range. If you raise the body size, raise this with it:
| Body size | A comfortable narrow width |
|---|---|
| 16px | 600–660 |
| 17px | 640–700 |
| 18px | 680–760 |
| 20px | 760–840 |
A serif face at the same pixel size usually reads a little narrower than a sans-serif, so nudge down if you have chosen one.
Picking a content width
1170px is the conventional choice and fits comfortably on a 1280px laptop with room for the scrollbar. Go wider only if you have something that needs the room — a full-width listing with big thumbnails, or full-width featured images.
Below about 900px the Full setting stops meaning much, since it will be close to the narrow column anyway.
Narrow can never exceed content
If you set a narrow width larger than the content width, it gives way: the theme uses the content width instead. A reading column wider than the page is not a thing, and the two settings save independently, so the rule has to be applied where both values are known.
The gutter
Outside the content width there is a 24px gutter on each side, dropping to 16px below the tablet width. It is not a setting. It is what keeps text off the edge of a phone screen, and every element that breaks out of the reading column measures against the same number, so nothing can disagree about where the page ends.