htmlpurifier/library/HTMLPurifier/AttrTransform/Textarea.php

<?php

/**
 * Sets height/width defaults for <textarea>
 */
class HTMLPurifier_AttrTransform_Textarea extends HTMLPurifier_AttrTransform
{

    public function transform($attr, $config, $context) {
        // Calculated from Firefox
        if (!isset($attr['cols'])) $attr['cols'] = '22';
        if (!isset($attr['rows'])) $attr['rows'] = '3';
        return $attr;
    }

}