It seems to be limited only by the script’s memory limit.
A quick test got me a key of 128mb no problem:
ini_set('memory_limit', '1024M');
$key = str_repeat('x', 1024 * 1024 * 128);
$foo = array($key => $key);
echo strlen(key($foo)) . "<br>";
echo strlen($foo[$key]) . "<br>";