Maybe I'm not going it right? I changed it, then went to a post and click on thumbnails but it seems to be the same size. I went back and edit the post again and then simply click ok to attach to post and still not change. Do I need to rebuild it or delete and upload again?
I don't see any option to rebuild attachment thumbnails in the current beta. The only way I see to generate a new thumbnail is to upload the image again.