You can use something like jQuery Simply Countable plugin and attach it to excerpt input.
Limit_Excerpt_Words::on_load();
class Limit_Excerpt_Words {
static function on_load() {
add_action( 'admin_enqueue_scripts', array( __CLASS__, 'admin_enqueue_scripts' ) );
}
static function admin_enqueue_scripts() {
global $hook_suffix;
if ( 'post.php' == $hook_suffix || 'post-new.php' == $hook_suffix ) {
wp_enqueue_script( 'jquery-simply-countable', plugins_url( '/jquery.simplyCountable.js', __FILE__ ), array( 'jquery' ), '0.4.2', true );
add_action( 'admin_print_footer_scripts', array( __CLASS__, 'admin_print_footer_scripts' ) );
}
}
static function admin_print_footer_scripts() {
?>
<script type="text/javascript">
jQuery(document).ready(function ($) {
$('#excerpt').simplyCountable({
countType:'words', maxCount:5, strictMax:true
});
});
</script>
<span id="counter" style="display:none;"></span><!-- needs counter to work -->
<?php
}
}
PS there is also word-count.js in WP core, but I can’t make sense if it’s as easy to use for blocking stuff.