Changing default header size

I’m so frustrated.

I’m working with theme Twenty Thirteen.

I’m trying to change the size of the header. I’ve looked up a million ways, including creating a child theme, and can’t get any of them to work.

The last thing I tried was to edit the custom-header.php file to change the dimensions for the input when you upload…but when I upload the image, it just shows it as blown up and then full-size on my site.

My site: https://www.mainstreetconstruction.net/dev