Make sure you have absolutely precise measurements because they are most particular about the pets having enough room to move around, but not being so big they become dangerous for the animal getting thrown around.
If you are using BA they have a link with the calculator ...
http://www.baworldcargo.com/biztools/pcst.shtml
P.S. Don't forget the Rabies jabs at least 6 months before travel, to avoid Quarantine (for pets
).