Crea un custom excerpt que corta el contenido por cantidad de caracteres, esta función tiene que escribirse en function.php Create a custom excerpt that cuts the content acording the number of charcaters define, Source: http://smallenvelop.com/4-ways-to-limit-post-excerpt-length-in-wordpress/

custom_excerpt.php

<?php
function get_excerpt(){
$excerpt = get_the_content();
$excerpt = preg_replace(" (\[.*?\])",'',$excerpt);
$excerpt = strip_shortcodes($excerpt);
$excerpt = strip_tags($excerpt);
$excerpt = substr($excerpt, 0, 50); // characters number
$excerpt = substr($excerpt, 0, strripos($excerpt, " "));
$excerpt = trim(preg_replace( '/\s+/', ' ', $excerpt));
$excerpt = $excerpt.'... <a href="'.$permalink.'">more</a>';
return $excerpt;
}