How to set lightning:datatable fixed column widths

You can use the initialWidth property of the column to set the width when it is rendered. It can still be resized.

[
    { label: 'Message', fieldName: 'Description', type: 'text' },
    {   
        type:  'button',
        initialWidth: 34,
        typeAttributes: {
            iconName: 'utility:new_window',
            name: 'newWindow', 
            title: 'Popout Window', 
            disabled: false, 
            value: 'test'
        }
    }
]