To change the the size in width or height or both of the header of  Twenty Eleven or Twenty Ten theme in WordPress you need to edit the ‘functions.php’ file in your theme or child theme – the preference is to make a child theme and create/change the  functions.php file in there.

The reason for this is that if the main theme is updated you will lose your custom changes.

The functions file can be found as below:

/public_html/wp-content/themes/twentyeleven/functions.php

/public_html/wp-content/themes/childthemeoftwentyeleven/functions.php

Add in the content below:

<?php
 
// The height and width of your header.
define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyeleven_header_image_width', 1000 ) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyeleven_header_image_height', 130 ) );
 
?>

What you need to change is the pixel dimensions for your required height and width. Now you can create a graphic to those dimensions and upload it through the front end WordPress interface.

Appearance > Header