The width of the first child element in a titlebar is always set to 'auto'.

Code:
Ext.define('App.view.StatusBar', {
    extend: 'Ext.TitleBar',
    xtype: 'StatusBar',
    config: {
        title: 'My title bar',
        docked: 'top',
        items: [
        { hidden: true },
        {
           xtype: 'button',
            action: 'home',
            iconCls: 'orb',
            ui: 'orb',
            width: 60
        }
    ....
My workaround is to add a hidden element as first child (red code).