Airline seats come in a variety of sizes and configurations depending on the aircraft and the airline. However, there are some general guidelines regarding seat size and what may be considered too big for an airline seat.

The standard width of an airline seat is typically around 17-18 inches, with a pitch (the distance between the back of one seat and the back of the seat in front of it) of around 31-33 inches. However, some airlines offer seats with more legroom or wider seats in premium cabins, such as first or business class.
For passengers who are particularly tall or broad, fitting comfortably into a standard airline seat can be a challenge. In these cases, some airlines may offer the option to purchase a second seat in order to ensure that the passenger has enough space to sit comfortably. Alternatively, airlines may offer seats with extra legroom or wider seats in certain rows or sections of the plane, which can be booked for an additional fee.
In recent years, there has been growing concern about the size of airline seats and the impact that they can have on passenger comfort and health. Some studies have suggested that the cramped conditions of airline cabins can increase the risk of blood clots, deep vein thrombosis, and other health issues.
In response to these concerns, some lawmakers and consumer advocacy groups have called for regulations regarding airline seat size and pitch. In 2018, the Federal Aviation Administration Reauthorization Act was signed into law, which directed the FAA to establish minimum seat size standards for commercial airlines. However, as of 2021, no such standards have been established.
In the absence of formal regulations, many airlines have taken steps to improve passenger comfort by offering wider seats, more legroom, and other amenities. However, the availability of these options can vary widely depending on the airline, the aircraft, and the fare class.
Ultimately, what may be considered too big for an airline seat depends on a number of factors, including the size and shape of the individual passenger, the width and pitch of the seat, and the duration of the flight. For passengers who are concerned about fitting comfortably into an airline seat, it may be helpful to research the options available on different airlines and to consider booking seats with extra legroom or purchasing a second seat if necessary.