The amount of space under a seat varies slightly from airline to airline, and different planes also have different under-seat dimensions. For a Delta Airlines Boeing 757-300, the under-seat dimensions are 20 inches wide, 17 inches deep and about 11 inches tall.
For an American Airlines 737-800, the under-seat dimensions are 20 inches by 17 inches by 11.75 inches. A Continental 737-800/900 has slightly less room than the same American Airlines plane. A Southwest Airlines 737-800 has under-seat dimensions of 19 inches by 19 inches by 9.5 inches, and in a United Airlines 757-200, the dimensions are 17 inches by 20 inches by 10.5 inches. However, it is possible for the airline to prohibit a piece of baggage that fits these dimensions.