How about we just create a tutorial right here 🙂 The ones I found were over-complicated.
So, locate this section in your functions.php file:
// Add support for custom header add_theme_support( 'custom-header', array( 'width' => 300, 'height' => 80, 'header-selector' => '.site-title a', 'header-text' => false ) );
And change that width and height to whatever dimensions you like. You’ll likely need to change a few things in your style.css file but see how it looks first and then I can give you tips on that.
Remember it is recommended to have FTP access to your site to make the changes to the files. One mistake in the Appearance > Editor section of your dashboard and you can render your site inaccessible. Also it’s a good idea to save a backup of any of the files you are making changes to.