Memcache by default will not accept objects larger than 1M. This script looks through all the Drupal cache tables (provided they're named correctly) and discovers any objects that would blow this requirement. Table names cannot be prepared as these are not variables that cannot be altered at runtime without changing the meaning of the query. It …

cache_plotter.php

<?php

$user = '';
$pass = '';
$database = '';
$limit = 1000000;

$dsn = 'mysql:dbname=' . $database . ';unix_socket=/var/run/mysqld/mysqld.sock';

try {
  $dbh = new PDO($dsn, $user, $pass);
} catch (PDOException $e) {
  echo 'Connection failed: ' . $e->getMessage();
}

$ctables = $dbh->query("SHOW TABLES LIKE 'cache%'",PDO::FETCH_NUM);
while($result = $ctables->fetch()) {
  $caches[] = $result[0];
}

foreach ($caches as $key => &$cache) {
  print "Checking cache: " . $cache . PHP_EOL;
  $sth = $dbh->prepare("SELECT cid, length(data) length FROM " . $cache . " WHERE LENGTH(data) > ?");
  $sth->execute(array($limit));

  $result = $sth->fetchAll();
  foreach ($result as $record) {
    print "cid: " . $record['cid'] . PHP_EOL;
    print "length: " . $record['length'] . " bytes" . PHP_EOL;
  }
  print PHP_EOL;
}