Twenty eleven-change header image size

Add the following code to the `style.css’ of your CHILDtheme:

#branding img {
    height: auto;
    display: block;
    width: 10%; /*play with these values*/
    margin:0 auto; /* if you want to center the result*/
    float:right; /* if you want to float right ... or left*/
}

You don’t have a childtheme yet?

1. Install and activate ‘one click childtheme plugin
2. create the childtheme
3. activate the childtheme
4. Make the changes in the CHILDtheme!
   – copy php files / css-file(s) of the original theme [as required] to the childtheme directory and edit them as required.

example:

function twentyelevenchild_custom_header_setup() {
	$header_args = array( 'height' => 175, 'width' => 1000 );
	add_theme_support( 'custom-header', $header_args );
}
add_action( 'after_setup_theme', 'twentyelevenchild_custom_header_setup' );

Thanks alchmyth,
I add your example to my functions.php file in my child’s theme and it actually displayed the above code on the page!?
No change to the header size though.

the basics:
https://codex.www.ads-software.com/Child_Themes#Using_functions.php

a brand new functions.php needs to start with this in the first line:

<?php

(absolutely nothing before that)

and it might end with:

?>

(not necessary; but when, then nothing after that)