The container shall be large enough to permit the animal to stand in a natural position, turn around and lie down (see the proportional dimensions specified in the illustration). For transportation of more than one animal in the same container, the dimensions should relate to the largest animal. For transportation of 2 animals in the same container, the width of the container should be calculated as 3 x C, for 3 animals in the same container the calculation is 4 x C. The guide lines mentioned above are from the IATA Live Animal Regulations book.

Why PAN uses as well the sitting position
Most of all dogs when owner wants to measure will lower the head. Thus giving a false height size. A sitting position is a natural height.