WordPress 3.5 not getting thumbnail

I have WordPress 3.5.

I want to display the thumbnail 150×150 pixel, but when I have the value of “thumbnail” in “Post’s thumbnail custom field name” it comes back with the full 620×350 pixel image.

Not sure what to do.

If you have WordPress version 2.9 or higher, you can use built-in thumbnails support – RSS Manager will fetch them automatically.

How exactly do I take advantage of the built-in thumbnail support? Am I to do anything besides enter “thumbnail”?

http://ww.wp.xz.cn/extend/plugins/rss-manager/