How do I add new layout width options in Wordpress editor?

Question

How can I add new custom widths to the block alignment menu?

I have set the two inbuilt ones in my theme.json, but how do I add more? Eg. narrowSize etc.

{
    "settings": {
        "layout": {
            "contentSize": "1200px",
            "wideSize": "1920px"
        }
    }
}