Fields can be constructed in a variety of formats, such as: text inputs, select boxes, checkboxes, radio buttons, and multi-select. You can specify a field’s format by passing a ‘format’ parameter to the field’s arguments:

$args['fields'][] = array('type' => 'taxonomy', 'format' => 'select');

The above code will create a taxonomy field as a select box. For more information, consult the documentation on field formats parameters.