Question: what size are atc cards?
Answer: Artist trading cards (ATCs) is a conceptual art project initiated by the Swiss artist M. Vänçi Stirnemann in 1997. He called it a Collaborative Cultural Performance. Artist trading cards are 2 1⁄2 by 3 1⁄2 inches (64 mm × 89 mm) in size, the same format as modern trading cards (hockey cards or baseball cards).