"Poker size" cards: layout for easy printing
US Poker sized cards are 2.5" x 3.5". Of course, your art should be slightly larger than that if you want to have printing to the edge of the card. How much larger will depend on your print house, but 1/8" on each side (so 1/4" total in each dimension) should suffice, making the art 2.75" x 3.75".