You can do File >> Export as New File wherein you get to choose file size to export. Usually thumbnails are like.. 240x360 i guess... so you can choose the same and see if that serves your purpose?
The thumbnails are stored in the catalog folder in the thumbs.5.cache file. It's an sqlite database which stores the thumbnails as a special binary type field. I don't see how you could extract the thumbnails either from the Organizer or from the cache file.
The only way would be to use the Organizer to show each file, make a screenshot and save...
Sorry for the bad news.
Thanks to you 00jon, (and TimeMachine).
The SQLite app I found is a table (index), but i cannot see the actual pictures. Any ideas on how I can see and/or export them? I know I can probably open them in Adobe, and capture screen shots, but that could take a very long time (I think there are a few thousand pictures).