I should have been clearer.
It is the feature image that I would like to size. It is inserted full size and WP doesn’t offer any option to size it so far as I can see.
I need to be clearer.
If I have an image, say, 1000px x 500px and I use it as the feature image it is too big.
I could make a smaller version to use as the feature image but I was hoping there is a way to size it in the same way you can size the page header instead of having to create a specific image to use as the feature image e.g a soft crop to a size.
You either have to resize it manually, or upload it in the Page Header metabox and set the size (this will display the same as the featured image if the featured image is empty).