By default, WordPress gives you the full browser width to work with – no automatic content restrictions. Every block uses flow layout, taking 100% of its container width, which isn’t ideal for readable content.
Content size solves this by setting a default width constraint that applies automatically in the post editor. Define it in theme.json under “settings > layout > contentSize” using any valid CSS value.
Wide size adds a second, larger width option that appears as an alignment choice for blocks. Users can choose between default content width, wide width, or full width depending on their needs.
These settings immediately apply in the post editor for content creation, but the site editor requires you to explicitly enable them on container blocks – giving you more layout flexibility during theme development.