As you may imagine, the size of the placemat you'll be able to obtain will depend upon the size of your table and the number of guests you're hosting. However, placemats need to be at least a certain size for an adult to have enough eating space!
An average placemat is sized 12x18 inches.
Another popular size for placemats is 14x20 inches – perhaps suited best for larger tables or for meals in which several utensils or plates will be required.
However, placemats do also come in many shapes. Rectangular ones are the most common, but novelty placemats (such as branded ones for youngsters) or oval-shaped or circular ones do have their place. Metallic circular ones are particularly lovely for festivities! In these cases, you'll still want to make sure that the placemat is at least 14-18 inches across at its widest point, to ensure that your guests have enough room to eat.