The header height is set in the functions.php file, on line 111.
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 198 ) );
Polite Suggestion
If you are doing changes to the twenty ten theme then consider creating a child theme and develop and test in a local environment.
There you would not change the default theme but add changes in the child themes functions.php
/** Tell WordPress to run child_theme_setup() when the 'after_setup_theme' hook is run. */
add_action( 'after_setup_theme', 'child_theme_setup' );
if ( !function_exists( 'child_theme_setup' ) ):
function child_theme_setup() {
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 320 ) );
}
endif;
If you are new to child themes here is a download for a twenty ten test theme, it has the starting files including the after_theme_setup function above without the code.
Just open the style.css and change the theme name uri etc:
HTH
David