I tried
<?= $form->field($model, 'q', array('class' => 'form-control input-lg')) ?>
and
<?= $form->field($model, 'q', ['class' => 'form-control input-lg']) ?>
It gives the following error
ReflectionException Class form-control input-lg does not exist
The docs were no help and didn't give examples.
http://www.yiiframework.com/doc-2.0/yii-widgets-activeform.html#field()-detail
The guide even shows the above format for a button!