OK, I have found solution – “rows” parameter in this line:
$this->elements_html .= ‘ <textarea name=”‘.$id.'” class=”text_area ‘.$element_class.'” cols=”40″ rows=”7″ id=”‘.$id.'” >’.$value.'</textarea>’;
in class-form-generator.php file located here: ./wp-content/themes/enfold/framework/php/
But the question is how to embed this change into child theme?