ACEO stands for Art Cards Editions and Originals. They are the same size as baseball cards, except they are minature pieces of art. All of them have the same measurements: 2.5" x 3.5".
ACEOs are collectible little pieces of art. An ACEO is always two and one-half inches by three and one-half inches. That is the size of a standard sports trading card. The rule about size is the ONLY rule in the ACEO world. An ACEO can be created in any medium the artist desires: paint, colored pencils, ink, etc. There are even ACEOs made from wood, clay, fabric, and metal.
