Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 55900)
+++ wp-includes/general-template.php	(working copy)
@@ -1100,7 +1100,7 @@
 		 * If the alt attribute is not empty, there's no need to explicitly pass it
 		 * because wp_get_attachment_image() already adds the alt attribute.
 		 */
-		$image = wp_get_attachment_image( $custom_logo_id, 'full', false, $custom_logo_attr );
+		$image = wp_get_attachment_image( $custom_logo_id, array( 512, 512 ), false, $custom_logo_attr );
 
 		if ( $unlink_homepage_logo && is_front_page() && ! is_paged() ) {
 			// If on the home page, don't link the logo to home.