It must be large enough for your pet to stand and sit straight, to comfortably turn around and to lie down in a natural position. The back and sides of the container should have ample air holes for the comfort of your pet. The front of the container should have an appropriate welded wire mesh grille.
Size is essential for safety. It is not just essential but it is International Law that all animals are shipped in an IATA approved flight container that offers them comfort, safety and adequate ventilation during their flight.
For cats and dogs you must ensure that they have enough room to stand up straight, turn around and stretch out inside the container.