While theme.json sets global content and wide sizes, you can customize these values on individual blocks through the site editor interface. Each Group block can override the default settings when needed.
However, these custom sizes don’t inherit – child blocks always use the global theme.json values unless explicitly overridden. This prevents unintended cascading but requires thoughtful planning.
For post editor content width, edit the template that contains it. The Post Content block in your single post template controls the content width for all post editing, and different post types can have different templates with different settings.
Best practice: minimize custom content sizes and use them consistently. Having too many different widths creates visual chaos – stick to your global settings and only override when truly necessary for your block theme design.