The textarea field type should be any text-like type in your database.
'name' => array(
'type' => 'textarea',
'title' => 'Name',
'limit' => 300, //optional, defaults to no limit
'height' => 130, //optional, defaults to 100
)
In the edit form, an admin user will be presented with a textarea.
The limit option lets you set a character limit for the field.
The height option lets you set the height of the textarea in pixels.