Reply To: how to change image heights in header?

Add this custom CSS at ‘Appearance’ > ‘Customize’ > ‘Additional CSS’

/* larger screens */
.entry-header.with-image,
.archive-header.with-image {
    min-height: 30rem;
}

/* mobile screens */
@media (max-width:768px) {
    .entry-header.with-image,
    .archive-header.with-image {
        min-height: 20rem;
    }
}

Change the min-height values to change the height.
You can use px instead of rem if you prefer e.g. min-height: 400px