The default Twenty Seventeen post and page featured images appear quite large in height especially on a laptop or computer screen. The height of these featured images can be reduced in size.

The default height of single post and page featured images.

A reduced height single post and page featured image.

The height of the featured image in this example has been reduced in size to 33% of the height of the screen.

Where and how to change the height of single post and page featured images:

In WordPress Admin > Appearance > Edit CSS.

Enter the CSS code below.

The 33vh height sets the size of the images to 33 percent of the height of the screen.
The object-fit line ensures that the image retains it's original proportions and is cropped to fit the shape of the featured image bounding rectangle.

The CSS code:

.single-featured-image-header img {
	/* display: block; */
	/* margin: auto; */
	height: 33vh;
	object-fit: cover;
}