Originally Posted by
mtofell
As they've introduced seatback power, etc. there are various equipment boxes that encroach into the space under various seats. It varies a lot between aircraft and individual seat so I think OP is going to have a tough time answering this question. Also, keep in mind they want the bags to fit all the way under the seat for quick evacuation so if OP is hoping to leave the bag hanging out as a footrest during takeoff/landing that's not going to be an option.
I think that this is really it. The answer varies by seat to seat, even on the same aircraft.
I think OP's best bet is to get something that meets the "personal item" dimensions posted above. Then it will always fit, no matter the aircraft or the seat.