I’m going to give all the details I can.
I set my image sizes – Max 2000 x 2000 Medium – 800 x 0 thumbnail 250 x 150
I worked out all the different sizes that would resize to 250 x 150 correctly.
I uploaded an image that was 2000 x 1200
I did a blog post and hit Add Image – Attachment Details came up and it had Attachment Settings. All the sizes were available – Full large medium and thumbnail – so that worked like a charm.
I picked Medium ( 800 x 480 and hit Insert the phot. worked great, fit perfectly.
Then I went to Featured Image (on the right side of the editor) clicked Set Featured Image – it took me to my image library.
I clicked on the full image ( the original one I uploaded. (that’s the only size showing in my library, none of the other sizes are there).
The size of the image is 2000 x 1200 – there is nothing to choose the thumbnail size. If I hit publish it uses the biggest that will fit on the index page. I’d like it to use the thumbnail size.
I have Customizer > Layout > Page Header > Single Post Header Location – Hide ( no idea if that’s important)