echo  mb_strlen(serialize((array)$arr), '8bit');

i don't have a solution to check the size of every variable, but if you use doctrine its probably the reason

you need to use

   $Elem->free(true);

another thing is to upgrade to 5.3 (if you dont do it yet), the garbage collector of 5.3 is better

Well, since Memcached doesn't store raw objects (it actually stores the serialiezd version), you can do this:

$serializedFoo = serialize($foo);
if (function_exists('mb_strlen')) {
    $size = mb_strlen($serializedFoo, '8bit');
} else {
    $size = strlen($serializedFoo);
}

I believe PHP's memcache implementation uses serialize when storing in memcached. You can simply serialize the output and check it's size:

<?php
$data = array('foo' => 'bar');
$serialized_data = serialize($data);
$size = strlen($serialized_data);
# `strlen` returns number of chars in a string. Each char is 1 byte.
# So to get size in bits, multiply `strlen` results by 8. Divide by
# 1024 for KB or KiB. Divide by 1000 for kB.
print($size * 8 / 1000);
?>

You can Use MultipartCache it's a simple class that extends memcache to help you split the arrays or string automatically

Simple Test

$cache = new Mcache\Main();
$cache->addserver("127.0.0.1");  // Local memecache server

$cache->set($key, file_get_contents("large_image.jpg"));

header("Content-Type: image/jpeg");
echo $cache->get($key); // large image for cache 

If you are just interested in the size of of your data then

$data = serialize($this->results);
echo strlen($data) . " bytes";