|CC licensed content portal: http://www.flickr.com/creativecommons/ also CC-enabled search at http://flickr.com/search/advanced/|
|CC licensed content feed: ?|
|Approximate size of the CC-licensed collection:|
Flickr's choose license implementation can be seen at http://flickr.com/account/prefs/license/ (you must be logged in to a Flickr account).
Flickr's implementation is of the drop down menu variety.