Best way to rescale default WP 2017 theme header image size?

I have been using the WordPress 2017 theme. I like it. It works well. But I wish the default image wasn’t 2000 px X 1000 px. It’s too overwhelming.

What’s an easy way of changing the default image size? If I simply upload a smaller image it scales up and looks pixelated.

In particular I’d like the height to be 300 px or so.

Thanks,

doug