You'll need to decode into PHP arrays before you do any work with the data.

Try:

$hugeArray = json_decode($huge, true); // Where variable $huge holds your JSON string.

echo count($hugeArray);

If you need to count to a lower depth, you'll need to iterate through the array.

For example, if you want to count the number of elements in the next layer, you can do:

foreach ($hugeArray as value) {
    echo $key.' - '.count($value);
}

However, this isn't necessarily meaningful because it depends on what you are trying to count, what your goal is. This block only counts the number of elements 1 layer underneath, regardless of what the actual numbers could mean.

You've declared an associative array, not an indexed array. So you can't use length.

To get the count from an associative array, you need to iterate through its keys:

var count=0;
for (var key in $invSlots['slots'])
    count++;

I guess this will be useful for many others, so I decided to answer to my own question using @mega6382 solution:

// prepare the JSON array 
$JSONDATA= 
array(
    'response' => true, 
    'error' => null,
    'payload' => 
        array(
            'content' => $content,
            'size' => $size 
            )
    );
// evaluate the JSON output size WITHOUT accounting for the size string itself
$t = mb_strlen(json_encode($JSONDATA, JSON_NUMERIC_CHECK), '8bit');
// add the contribution of the size string and update the value
$JSONDATA['payload']['size']=$t+strlen($t);
// output the JSON data
$joutput=json_encode($JSONDATA,JSON_NUMERIC_CHECK);
echo $joutput;

the very simple solution is decode the json and you can now check if the decoded json is an empty array.

$json = file_get_contents('php://input');
$json = json_decode($json);// decode json`
if (empty($json)) {
   // json is empty
}

If I understood well your question, you need the size of each "block" subarray inside the main array.

You can do something like this:

$sizes = array();
foreach($returnedArray as $key => $content) {
    $sizes[$key] = count($content);
}

The $sizes array will be an associative array which the various "block"s as keys and the size of the data as values.

Edit: after the edit of the question, if the data inside the innermost arrays are strings or integers you can use a function like this:

function getSize($arr) {
    $tot = 0;
    foreach($arr as $a) {
        if (is_array($a)) {
            $tot += getSize($a);
        }
        if (is_string($a)) {
            $tot += strlen($a);
        }
        if (is_int($a)) {
            $tot += PHP_INT_SIZE;
        }
    }
    return $tot;
}

assuming to have only ASCII-encoded strings.