Every exchange I've participated in has been 100% international shipping for me. So what? That's the price I have to pay to be on any kind of exchange.
As to sizes and mounting/matting:
I usually print on either 8x10" or 9.5x12" paper, with somewhat varying margins depending on film format and whether I want to crop anything.
I have learned that a well packed unmounted print has a far better "survival rate" in international shipping. Mats tend to arrive damaged, too. So I will ship mine unmounted and unmatted.
Since 8x10" is far easier to fit in a reasonably standard-sized padded envelope that's probably what I will use. Unless the picture just wants to be bigger, of course...