Depending on the sizes of the photos you are sending, you choose the size of the envelope. If you are sending a 4*6 photo, then you can just use a regular business envelope. But the most commonly used one is a 9*12 manila envelope. You can get them easily at stationery stores.

Are you sending just a letter requesting a photo? You really don't have to send any SASE (unless you want to send just a piece of paper and an SASE in a 9x12 envelope, then in that case it's better just to include a photo.)

However, if you're including your own photo, pick the same size you are sending the photo in and fold it in half and send it to the person you wish to have an autograph from.

Depends which country you're writing to, in the UK if a celeb supplies their own photo it will usually be 6" x 4" (I've had two 10 x 8s, but in those cases the person I sent to supplied their own envelope as well!) In the US they tend to be bigger, as charliestcloud says.

You will probably get a better success rate if you enclose something to sign, even if it's just a blank index card.

It depends what size the photo you're sending is.......But if you want them to send you a photo, they are usually an 8x10 or 8.5x11, there not usually bigger than that......

I am just starting collecting autographs and I was surfing around the site and I heard about SASEs. How do you know what size SASE to send for your autograph?