ATCs, or Artist Trading Cards, are miniature pieces of art that are swapped among artists. There are very few rules in creating ATCs, but there are two that are super important. First, ATCs are always traded and never sold. The second, equally important, rule is the size. An ATC should always measure 2.5″ x 3.5″. They can be positioned vertically or horizontally, although some artists prefer them portrait style because they store them in binders using protective sleeves such as those used for baseball cards.
There’s a quick video by artist Bob Rankin that gives a great history and overview of ATCs. You can find it here ATC video