What size underseat pet carrier fits most airlines?

Most U.S. carriers cap soft-sided underseat bags around 17–18 inches long by 11 inches wide and roughly 11 inches tall, which is why the Vceoa (17.5x11x11), Henkelion, Petskd (18x11x11) and Sherpa medium sizes show up repeatedly. Reviewers consistently warn that dimension limits vary by airline and even by aircraft, so confirm your specific flight's underseat clearance before buying.