First of all, the size of your travel cage will depend on the size of your pet.
With this in mind, you need to take certain measurements according to the below schema:
Blue: from the snout to the end of the tail
Red: from the end of the elbow to the ground
Yellow: width of the body at your pet’s widest point
Green: the standing height of your pet, at the highest point (from the ground to the top of the head or the tips of the ears)
Next, breed, body shape and even weight can influence the choice of cage.