Many thanks for your quick response.
This works – except that the image in the post itself is now also conforming to that thumbnail size, and I want it to be full size. Here is a blog that shows what I want: http://www.healthyhomecleaning.com/
See how the thumbnails are all squares of the very same size? It gives a nice, uniform look. Then if you go to the actual post, you can see that the image is full-size.