Hi Reo,
As KBH said, it's about four sheets of paper that they send back.
The main thing for me, mailing from the US, was that I wanted to be sure that the FedEx return envelope was properly labeled. So I used a FedEx "letter" sized envelope (technically it's a little bigger than letter) as my return envelope. And then I put that, along with the PP and photographs into a FedEx "legal sized" envelope. The nice thing about the FedEx legal size is that it actually has some give in the sealing flap to accommodate the FedEx letter envelope.
Also, I totally agonized over this too, and I just want to let you know, if you've already sent something and it's a different configuration from what I sent, it will still all be okay