Works great with twenty twelve

Hi brocchini,

Thank you for taking the time to rate the plugin!

WP Display Header has no influence on the size of the header images, it only changes which image to use. You can determine the size in the header upload dialogue under Appearance > Header, by resizing the cropping box:
http://en.support.wordpress.com/themes/custom-header-image/#uploading-a-custom-header-image