According to About.com - Painting, an ATC must be 2.5x3.5 inches or 64x89 mm as that is the size of the original sports trading cards. The only other requirement that I've read about is that they are traded, not sold. I read about them here. It says that they have to be exactly the right size and made on card stock, so we could have an exception - the orig size is the same, but we start on fabric or our choice AND if anything goes over the edge to make the card slightly bigger, that's okay (beaders do that!).
Have a Beady Good Day. Karen