Your pets need to travel in containers that conform to IATA standards. Your pets must be able to lie down, stand up and turn around comfortably in their travel container(s). It is important to get the measurements right for your pet’s comfort and safety, as they could be refused travel if the container isn’t the right size.
Another factor to take into consideration would be one of price. Pets travel in the heated hold of passenger aircraft. The charge is based and calculated upon the amount of space your pet’s travel container occupies i.e. it’s volumetric weight. An over sized container means unnecessary additional cost.