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.