If this is a US Airways 330, the two aisle seats in the section with four seats (the middle part of the aircraft) do NOT have enough room for a pet carrier. They barely have enough room for my feet actually.

I would personally try to get either of the seats in the side sections...either window or aisle. Speaking from personal experience, there is less room in the aisle seats in the four seat section than the aisle seats in the two seat section.

Please trust me on this one, though I don't believe you are on US Airways.

You may want to go to the forum for the airline you are considering and ask your question there as well. While related to pets, there is an element of the quesiton that is airline specific. I'd say the question to ask is "which seats in coach on this aircraft have the most legroom" and you will get a lot of helpful information.