So I am planning a trip to Cancun and have been asked to transport a dog (dashund)... the owner is currently in Cancun....
What's the largest collapsable carrier I can get away with in Biz class Airbus A321neo

I know what AA allows for pet carrier size.... HOWEVER on the AA site it states "Soft-sided collapsible kennels can be slightly larger but still need to fit under the seat without having to excessively collapse the kennel, have to be secure, padded, made of water-repellant material and have nylon mesh ventilation on 2 or more sides". (any other airline with a more liberal policy??)

So what's the size of a collapsable that I can get away with????