You can go here and check the requirements for your specific airline:
http://www.pettravel.com/airline_rules.cfm
We are flying American and these are ther requirements:
Pet Carrier Guidelines
The maximum size for cabin pet carriers is 23" long x 13" wide x 9" high.
Animals must be able to stand up, turn around and lay down in a natural position in the kennel. Soft-sided pet carriers are accepted as long as they are constructed of water-repellent, padded nylon with mesh ventilation on two or more sides.
What drove me nuts was the part I have highlighted in blue above.