I always send my own A4-sized pictures, so I know precisely what envelope I need to get them back.
Only once I sent a letter to a celebrity who sends his own pre-signed pictures and doesn't sign those which people send to him. I simply researched which size that photo is and put the smallest envelope that it could fit into.

How about printing you own pictures to send off, that way you can get the right envelope to fit easily

I usually send a 9x12 (inches) sase because most send out 8x10's in USA. Best advice I can give you is do some research on the celeb your sending to and find out what size pics they send. Hope this helps

So I've always used IRC's to this day, but would like to switch to SASE's (I'm European). The guide on this forum states using 8 by 10, but calculating those in centimeters, isn't that rather big? Many autographs I've received were usually small enough for a standard envelope, so that's why I'm wondering a bit. It would seem a bit of a shame to send these rather expensive big envelopes (the difference in price between an 8 x 10 and a standard envelope is massive here), only to receive a picture that could've fitted in a much smaller one. So basically what I'm asking here is what size you usually send and the size of photos you receive. Or am I just really mixing up the sizes by using another metric system?

Sorry, bit confused here about what to do