You can do a mini if you like but just make sure it can be hung on a wall. It should be at least larger than a mugrug which is around 8" x 8". I don't like to put any numbers on it because then there is always someone who questions if theirs will be okay for this swap. So just think if the size would be something you would want for a wall hanging.
I just dont' want them to be the size of a coaster LOL