The only official rule is that the trading card measures 2.5" x 3.5". This line of artist paper packs feature downsized versions of the same Strathmore papers that artists use for their full-sized art, conveniently cut to the official trading size making it easy for artists to join this large and growing movement. The cards are available for a range of mediums drawing, watercolor, acrylic, oils and mixed media collage. The assorted pack contains two sheets each of vellum Bristol, smooth Bristol, canvas, textured and watercolor and one sheet each of illustration board and acrylic.