The maximum size for under-seat carry-on bags on American Airlines is 45 linear inches, and size restrictions vary depending on the airline and its plane model. The space under the seat of the American Airlines 737-800 plane is 20 inches wide by 17 inches deep by 11.75 inches tall.
The under-seat space on Delta Airlines' 757-300 plane is 20 inches by 17 inches by 11 inches. Delta's maximum carry-on bag size is 22 inches by 14 inches by 9 inches, or 45 linear inches. The space under seats on a Southwest Airlines 737-800 plane is 19 inches by 19 inches by 9.5 inches and carry-on bags cannot be larger than 24 inches by 16 inches by 10 inches. United Airlines' 757-200 plane has an under-the-seat depth of 17 inches by 10.5 inches by 20 inches. United Airlines prohibits carry-on bags that are larger than 22 inches by 14 inches by 9 inches.