Updated – the above Job image now displaying as Medium (300×300) from an original 512px x 512px image. I had to re-insert this image after changing default image sizes in Settings > Media.
And now, the above Grey image showing as Original/Full 300x300px from an original 300px x 300px
And now, the above Blue image showing as Original/Full 450x450px from an original 450px x 450px
And now, the above Red image showing as Original/Full 1200x1200px from an original 1200px x 1200px. It seems that this has been responsively rescaled to ensure it doesn’t skew the layout.
And finally, this green image is displayed as full size from an original of 800px by 800px. Note that the WP editor would suggest that the maximum container size is 840px, but actually the editor is different from the final published page with Aeolodicon Simple Life.
After a lot of trial and error, it seems like the real published container is approximately 760px wide. Note that on the iPhone, 300px is close to container width.
And final the above orange image is 750x750pxpx which I estimate is very close to maximum width after a lot of trial and error.