Making a 1140px wide header logo in the twenty-ten theme?

in your theme’s functions.php, look for this:

// The height and width of your custom header. You can hook into the theme's own filters to change these values.
    // Add a filter to twentyten_header_image_width and twentyten_header_image_height to change these values.
    define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyten_header_image_width', 940 ) );
    define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 198 ) );

Its best to creat a child theme, so you don’t loose your customization when you next update WP/theme.