Testing how wide this theme (McKinley) will display images. The answer is – not as wide as the previous theme I was using (Dalston), but I like the overall look of this theme better.
Update 12 December 2019
Changed to the TwentyTwenty theme and I now understand that some themes are built to take full advantage of the Gutenberg image sizing, while other themes are not. People new to WordPress might not realise this; I didn’t until I had played about with the other theme, and I have been using WP for years.