You can set the container width of the article here. Posts and pages use separate settings, you can find the settings for blog posts here:

Customizing ▸ Single Post Settings ▸ Container

The settings for pages is here:

Customizing ▸ Pages Settings ▸ Container

There are two presets for page width, Narrow and Normal.

Narrow Mode

In narrow mode, the content is restricted to the maximum width area, and you can set the maximum width yourself.

Normal Mode

The content and page width are the same.