To easily make a measurement, you measure from paw to the top of the head in a sitting positon next to you. Add then 8-10cm above the head and you will have the height of the crate. Further explanations are available on the link below

When selecting in the form the size of the crate, the crate sizes are generic and if you are close to the size shown you can select the one. Be careful with the size as this will determine the pricing structure, the higher the volume the higher the price. In opposite if you go to small, your pet will be refused and very high cost for changing will be necessary. If in doubt, contact the call center.

Make sure to contact the airline you are flying with to know the details of how they calculate the price you will pay once at the check-in. This may vary as they select volume and weight. Some airlines charge weight which means very high cost for your excess baggage. Do remember none of those charges are inclusive of your ticket, you will need to settle this upon check-in