perhaps this article from NYTimes is helpful.
https://www.nytimes.com/2017/01/09/t...age-rules.html Quoting the article :There is no universal carry-on bag size. Each airline has its own rules. And baggage allowances may vary across routes and cabin classes.
In 2015, the
International Air Transport Association, an industry group that represents about 260 airlines, created guidelines for optimum-size carry-on bags: 21.5 x 13.5 x 7.5 inches. But following a media hullabaloo, the guidelines were not widely adopted. Major airlines —
American,
Delta, United,
JetBlue Airways,
Cathay Pacific — have set their maximum carry-on size for economy passengers at 22 x 14 x 9 inches.
Some airlines have stricter rules or slightly different measurements (or a maximum of linear inches). For example, the maximum carry-on size for economy passengers on KLM Royal Dutch Airlines is 21.5 x 13.5 x 10 inches and Qatar Airways is 20 x 15 x 10 inches. Other airlines allow larger bags, most likely to the dismay of the Association of Flight Attendants-CWA, which has advocated for fewer and smaller bags “to reduce risks of injury and conflict onboard the aircraft.” For instance, British Airways allows carry-on bags up to 22 x 18 x 10 inches. Southwest Airlines allows carry-on bags up to 24 x 16 x 10 inches.
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I think IATA stepped away from the 21.5 X 13.5 X 7.5 rule as the currently website does say this.
https://www.iata.org/whatwedo/ops-in...check-bag.aspx