Index: wp-admin/network/site-new.php
===================================================================
--- wp-admin/network/site-new.php	(revision 21993)
+++ wp-admin/network/site-new.php	(working copy)
@@ -123,7 +123,7 @@
 			<th scope="row"><?php _e( 'Site Address' ) ?></th>
 			<td>
 			<?php if ( is_subdomain_install() ) { ?>
-				<input name="blog[domain]" type="text" class="regular-text" title="<?php esc_attr_e( 'Domain' ) ?>"/><span class="no-break">.<?php echo preg_replace( '|^www\.|', '', $current_site->domain ); ?></span>
+				<input name="blog[domain]" type="text" class="regular-text" title="<?php esc_attr_e( 'Domain' ) ?>" maxlength="63"/><span class="no-break">.<?php echo preg_replace( '|^www\.|', '', $current_site->domain ); ?></span>
 			<?php } else {
 				echo $current_site->domain . $current_site->path ?><input name="blog[domain]" class="regular-text" type="text" title="<?php esc_attr_e( 'Domain' ) ?>"/>
 			<?php }