The most popular size of a placemat is 14 inches wide by 20 inches long, which holds an entire place setting, according to Etiquette Scholar. However, placemats come in a variety of sizes and assorted shapes.
Large rectangular placemats are generally 12 inches wide by 18 inches long, while smaller placemats usually cover 12 inches in diameter. The smaller placemats come in circles, ovals or squares. To allow elbow room, it is suggested to leave a minimum of approximately 4 inches between each placemat. Placemats are also not recommended for a party of more than 10 people, because they create a spotty, crowded look on the table.