Is it thumb or full that is the problem?
Firstly, here is the codex page about the shortcode
http://codex.wordpress.org/Gallery_Shortcode
Notice that thumb is actually “thumbnail”
So its for the thumb image.
If you are having issues with then my initial thinking is that there are no images uploaded after you set the size in the post.
Try open the post again and upload fresh images and see if that works.