The size of the dog car cover depends on the size of your dog and the desired coverage area. It is important that the blanket be large enough to cover the area where your dog will sit or lie down, while still allowing sufficient freedom of movement.
The blanket should be large enough to cover the seat or back of the car seat and provide protection from dirt, pet hair and moisture. It should cover the entire seat and ideally also protect the edges and sides of the seat.