Max Size for Under Seat Luggage?
 

Does anyone know what the maximum dimensions are for something to fit in the space under most airline seats?

It could be pretty small (not much bigger than shoebox size). Really depends on the class and individual aircraft. I've been on a flight in first where you could (and my seatmate did) put a regular rollaboard under the seat. Some seating configurations have offset struts in three-seat configs. The struts are closer to the outside, so the space in front of the window/aisle seats has a strut dividing it. Then there are the many longhaul planes that have entertainment boxes under a great many of the seats. If you're unfortunate enough to get one of those, space is very limited indeed.

AA is the only airline that defines a personal bag that I know of and I find their dimension reasonable. I have a personal bag that fits this dimension perfectly and I have had no problems with fitting in underneath the seat in front of me. So far no other airline has challanged this bag size either.

Shak

Personal item - includes: small backpack purse, briefcase, laptop bag OR a similar item such as book bag or tote that does not exceed 36 inches( length+width+height) and must fit under the seat.

Varies a lot, even within the same seating class in the same aircraft. For example, in 737 economy class on any airline that uses those airplanes, the window, middle, and aisle under seat spaces can be different sizes.

The window seat under seat spaces are often oddly shaped because of the curvature of the fuselage. As previously stated, some non-window seats may have some space taken up by in flight entertainment equipment.

There is also variation due to airline policies on how far under the seat stuff put there has to be (based on airline interpretation of how far sticking out is a hazard for someone climbing out in an emergency).