I usually send 4x6's or 5x7's. They turn out lovely signed, and even personalised! (Even better)
I do however sometimes like to have the odd 8x10, for example from Peter Facinelli. But I dont send 8x10's, I just sometimes like the 8x10 the celeb sends out {thumb2}
I also send 4x6 photos for a few reasons,the main reason is they are cheaper to print and mail because you may never get your picture back so who wants to spend the extra on ink and postage,also with the smaller pictures less chance of them getting bent or damaged.And as long as you pick your picture wisely there is lots of room for the autograph and even a inscription.
When I send photos, typically I send 4x6 photos. They fit perfectly in photo albums (and it's super easy to store them that way too!) Plus they save a ton of money versus the 8x10's
I use 4x6 photos. All of my autographs are stored in a photo album and they look lovely I have a few 8x10 and 5x7 photos but they are ones that the celeb has provided for me.
Again 4x6 are cheaper to print and cheaper to mail {thumb2}