This question has been answered by sottwell. See the first response.
case 'description':
$rows = ! empty($this->props['descriptionrows'])
? $this->props['descriptionrows']
: '10';
$cols = ! empty($this->props['descriptioncols'])
? $this->props['descriptioncols']
: '60';
$inner .= $this->_displayTextarea($field,
$this->props['rtdescription'],
'np-description', $rows, $cols);
break;