Well, I'm no expert, but here's what I know... it's stored in
$modSettings['max_signatureLength']So, without me trying it myself, give this a shot...
Make a copy of Sources/Profile.php, then edit it...
Find:
// Make sure the signature isn't too long.
if (isset($_POST['signature']))
{
require_once($sourcedir . '/Subs-Post.php');
Add after that...
if ($context['user']['is_admin']) {
$modSettings['max_signatureLength'] = '99999';
}
but it may not work, who knows

It could be in ModSettings.php too.