Welcome to the forum StephaniePattinson
To answer your questions:
1. You will need an envelope big enough to fit everything you're sending.
2. The size of the SASE will depend on whether you have sent something to be signed or not. If you do send something, you will only need a SASE big enough to fit whatever you've sent to be signed. If you don't I suggest sending a large A4 (9x12 inch) envelope as this will fit both large and small photos.
3. The number of stamps will depend on the size of your SASE and the cost to post it. If you're in the US, check out their postal calcuator
here.
4. Yes. Put stamps on both your SASE and envelope. Make sure there's enough postage on each envelope.
5. Yes.
Good luck!
nowandagain [rockon]