Preparing cats for flight to Australia tomorrow - advice!

Hello!
Tomorrow my 2 cats fly to Australia - I'm a bit nervous about it!
Just want to know if anyone has some advice, just preparing the cages now.

I have quite big carry cages cos the cats are big - tall and a bit fat :)

The dude at the pet shop today showed me the standard carry cage for cats - way smaller than the ones I have. Just wondering, what happens at the airport? Do they take them out and measure them to make sure the carry cage meets the exact dimensions according to the diagram/animal measurements? Or just eyeball it to see if it looks fair?

Also, there's no way the cats will drink from one of those bottle feeder/nozzle things. I've bought a little insert tray to clip to the door - is that OK? Says it needs to be accessible from the outside - i think it kind of is because you could pour water in through the door?

And, how early before the flight should I realistically go? Having trouble getting an answer from cathay today.

I feel really nervous!

cheers!