Kim O. is correct regarding the FAA guidelines for domestic flights, and as long as you have a car seat that is FAA certified to be used on an airplane rear facing, it's okay. International travel is a different story. Domestic airlines will still follow the FAA guidelines, but the European airlines have different guidelines and, I've found, Asian, or specifically, Chinese, airlines have no guidelines and don't seem to care.
I've traveled with my Peg Perego Viaggio rear facing infant car seat with no problems within the US, but in Europe, British Airways wouldn't allow it. They only allow Britax forward facing car seats. I think many European airlines don't allow rear facing (funny since while driving in Europe, they recommend keeping your children rear facing until 4 years of age!) I've traveled with my Britax Marathon and Boulevard convertible car seats with no problems within the US or Europe.
To Asia, with Air China and with China Eastern, was a different story. I'm Chinese, so I'll say this, but I think Chinese are too cheap to purchase a plane ticket for our babies, so when I did for mine, the airline workers couldn't believe it and kept telling me that there was no room for the car seat and that I'd have to check it. I had to keep telling and showing them, in English and in Chinese, that my baby had a purchased seat and I was d*** well going to keep my child safe and use the car seat. Once on the planes, flight attendants didn't hassle me, were cordial but useless regarding my car seat. I had to ask for a seat belt extender and tell them what to do.
If you are worried about the car seat fitting, the smaller Britax car seats should be fine as well as the suggested seats of the other moms. Something I highly, highly suggest is getting a GoGo Kidz Travelmate or a Traveling Toddler Accessory or something similar to lug your car seat thru the airport!