Millions of fliers might soon want to buy new carry-on suitcases.

The "optimal" carry-on size at 21.5 inches tall by 13.5 inches wide by 7.5 inches deep.

Global airlines announced Tuesday a new guideline that recommends shrinking carry-on bags, in an effort to free up space in packed overhead bins.

The guideline, which is not binding, means that many existing bags currently in compliance with airline rules would not be given preferential treatment in the boarding process. While details of how the guideline will be implemented are murky, and could vary from airline to airline, it raises the possibility that many fliers would be forced to check their favorite carry-on bag.

Fliers might either need to buy smaller suitcases or pay a fee to check their bags, typically $25 each way.

The recommendation by the International Air Transport Association suggests an "optimal" carry-on size at 21.5 inches tall by 13.5 inches wide by 7.5 inches deep. That's smaller than the current maximum size allowed by many airlines. For instance, American Airlines, Delta Air Lines and United Airlines all currently allow bags up to 22 inches by 14 inches by 9 inches -- although gate agents don't always enforce those more-generous measurements.

Airlines around the globe have varying standards -- different enough that a carry-on bag that is acceptable to one airline isn't allowed in the cabin of another. The airline trade group says the new guideline will not necessarily replace each airline's rules on bag size, but gives them a uniform measurement that "will help iron out inconsistencies."

Theoretically, if airlines follow this guideline "everyone should have a chance to store their carry-on bags on board aircraft of 120 seats or larger," the trade group said. Today, it's typical for the last 20 or so passengers to board to be forced to check their bags at the gate because the bins are already full.

Eight major international airlines will soon introduce the guideline into their operations. Chris Goater, a spokesman for the transport association, said they are: Air China, Avianca, Azul, Cathay Pacific, China Southern, Emirates, Lufthansa and Qatar.

"It's certainly not mandatory," Goater said.

No U.S. airlines have yet signed on, but Goater expects more carriers to quickly do so. The suggested size was just unveiled publicly Tuesday at a meeting of global airline CEOs in Miami.

Goater said the airlines are working with several large luggage manufacturers but none that can be disclosed publicly. Bags with new labels, designating them as "Cabin OK," are expected to be in stores by the end of the year.