I'm fairly new to the ATC concept as well, having just done my first two this last month, so I'm no expert.
I did find that WIKI has an article on the subject.
http://en.wikipedia.org/wiki/Artist_trading_cardsArtist Trading Cards (or ATCs) are miniature works of art about the same size as baseball cards and thin enough to fit inside standard card-collector pockets, sleeves or sheets. The ATC movement developed out of the mail art movement and has its origin in Switzerland. The cards are usually traded or exchanged rather than sold.
M. Vänçi Stirnemann originated the idea in 1996, holding trading sessions in Zurich, Switzerland.
Clubs and trading sessions have largely replaced the original concept of trading the cards during individual encounters, and many ATC workshops end with a trading session.
While there are no firm standards that limit the size or shape of a baseball card, most cards of today are rectangular, measuring 2½ inches by 3½ inches (6.4 cm by 8.9 cm).
Here on drawspace.com, the two recent ATC activities I participated in, we used the postal service to send the card to each other. Personally I intend to find a fun way to display the cards sent to me from the other artists. Either a scrapbook, or even framed to hang.