The few times I flew with Yaz (and Eddie with companion) I used Sherba Bags. Yaz fit nicely in teh small one while big Eddie took the larger sized bag.

(this was before I knew of something called Pet Sitters. Funny, that's what I'm donig now! )

Sherba's fit comfortabily under the seat in front of you. The Cat (or most pets) will simply lie down or stretch out (Eddie slept through all flights. Yaz cried.)

They are pricey (maybe ebay has a deal or check your local pet stores for a used but in good condition model.) But it's a very good investment. It's much more comfortable taking Eddie (since Yaz left this earth in Feb, 2006) to the vet and other places instead of a bulky, heavier carrier.