For the sake of anyone looking this up in the archives, the solution to this problem is to check out the airline's policy on under seat pet carriers. Most airlines that allow pets will list the maximum size of a pet carrier on their website. For US, I'm sure the mainline aircraft will vary and all of the caveats mentioned previously will apply. However, I think this should be a pretty good guide.
US Airways accepts hard-sided carriers up to 17 inches long x 16 inches wide x 8 inches tall and soft-sided carriers up to 17 inches long X 16 inches wide X 10 inches tall. On US Airways Express flights, pet carriers must not be more than 15 inches long x 17 inches wide x 8 inches tall on CRJ aircraft, more than 16 inches long x 17 inches wide x 8 inches tall on DH3 aircraft or more than 15 inches long x 14 inches wide x 8 inches tall on BH1 aircraft.
WN does a pretty good job detailing their 737s and differentiates between the aisle and window seat, which has much less space than the middle seat. I found AA and United but Delta says to call their reservations desk.