Ideally you should upload images of all the same size and then theres no need for CSS or plugins or head banging 🙂
A simple option would be this CSS:
.single-post .featured-image.page-header-image-single img {
aspect-ratio: 16/9;
}
As it defines the aspect ratio as opposed to defining any fixed heights, all images will ultimately fill that space.
Adjust the ratio to whatever meets your needs.