it looks like the placeholder the plugin is generating is using the height of the full sized featured image ( eg. 800px ). Have you spoken to the plugin author how to change / fix that ?
I’m not sure what that option does, exactly, but I assume it would depend on the size of the featured image you’re using. You can choose the featured image size in Customize > Layout > Featured Images.
Its not as simple as that.
For example:
your example posts featured image original size is 1200 x 800px.
However it is then scaled down to fit the content and is displayed at 720 x 449px on desktop
On mobile the scaling down reduces it to 373 x 233px on a plus size phone.
So there isn’t a fixed size you can use.
Might be worth looking for a lazy load plugin that calculates the displayed size.
I am sorry but i don’t have any personal experience of WP Rocket – a lot of our users do use it – but i cannot recall if it loads a correct size placeholder.