Ext alert 设置宽度

Ext.Msg.show({
                                title : '错误提示',
                                msg : '添加失败',
                                buttons : Ext.Msg.OK,
                                minWidth:300,//设置宽度
                                fn : function() {
                                    unfield.focus(true);
                                    btn.enable();
                                },
                                icon : Ext.Msg.ERROR
                            });