Nevermind. A little digging through the source code and I found my answer.
This would definitely be helpful if documented somewhere, as well as maybe use the standard event such as LoaderEvent.AUDIT_COMPLETE or whatever.
Anyways, sorts me out and hopefully helps others.
loader.addEventListener("auditedSize", onAuditSize);
loader.auditSize();
private function onAuditSize (event:Event):void {
trace(loader.bytesTotal);
}