Define max page-width

How can I define a max page width? I tried several things like:

$theme = @{
    overrides = @{
        MuiGrid = @{
            root = @{
                maxwidth = '1600px'
            }
        }
    }
}

Probably a very simple question. Hope somebody can help me.

Product: PowerShell Universal
Version: 1.4.6