Artists' Trading Cards, also known as ATCs, are the same size of sports cards: 2.5" wide by 3.5" tall.
Artists do their thing on the card and then trade them amongst themselves.
No buying or selling, just exchanging ~ either in person or by mail.
(While some artists sell the same size art, sold pieces are not considered by most to be ATCs but ACEOs - Art Cards Editions & Originals.)

I made many of mine on poker-size playing cards and a few on baseball cards -
both excellent substrates with challenges of their own.

Click here to find out about the originator of ATCs
Click here for more info about how to make ATCs

           Rollover the thumbnails below to reveal larger images to the right.