Limit by word count excerpts.

word-limit-function.php

// Limit Word Count, Strip Tags from Result
function trunc($phrase, $max_words) {
   $phrase_array = explode(' ',strip_tags($phrase));
   if(count($phrase_array) > $max_words && $max_words > 0)
      ;
      $phrase = implode(' ',array_slice($phrase_array, 0, $max_words)).'...';
   return $phrase;
}

work-limit-markup.php

<?php $excerpt = get_the_excerpt(); 
echo '<p>' . trunc($excerpt, 15) . '...</p>';?>