var box1 = new Ext.form.Checkbox({
                        boxLabel:'A',
                        cls:'widthClass'
                    });
                    var box2 = new Ext.form.Checkbox({
                        boxLabel:'B',
                        cls:'widthClass'
                    });

<div id="form-ct"></div>
    <script type="text/javascript">
        Ext.onReady(function () {
            var box2 = new Ext.form.Checkbox({
                boxLabel: 'B',
                ctCls: 'widthClass',//设置容易form-ct的样式
                cls: 'widthClass', //设置input的样式
                listeners: { render: function (el) { el.getEl().parent().addClass('widthClass'); /*通过事件设置自动生成的input容器的样式*/} }
            });
            box2.render('form-ct');
        });
</script>