As the battle of the bags gets more competitive at the airports, fliers should be warned that the rules are not always as clear or as fair as they could be.
Veteran flier George Hobica of Airfarewatchdog recently learned that the hard way when his well-traveled carry-on was rejected at the gate by American Airlines.
Although his bag fit in the sizer, it stuck out one inch at the top, he said. Normally the bag is still allowed because the overall size of the bag is 43 linear inches, below the airline's stated 45-inch maximum. He was forced to hoof it back to check in, and he nearly missed his flight.
"When I got on the plane, there was a woman with my exact same bag in the overhead bin," he told CNBC. "Enforcement can be very arbitrary."
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Many of the major airlines limit carry-on bags to 45 linear inches: 14 inches wide by 22 inches long and 9 inches deep. Spokesmen for American, Delta and United on Friday all told CNBC they haven't changed those dimensions in years.
In March, United launched a very public campaign that it would begin better, more consistent enforcement of its existing carry-on limits. That change of enforcement has led to faster boarding of the planes, Charlie Hobart, a United spokesman said.
The request for more enforcement came at the request of fliers who had been complying with the rules all along, he said.
That should come as no surprise to fliers who are still mad about other passengers who try to get away with more than their fair share. A recent social media campaign to shame the big-bag toters has emerged with the hashtag #Carryonshame, which features images of the alleged offenders.
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All those rules about carry on sizes can be slightly different at each airline. Some other U.S.-based airlines, including Spirit, Frontier and the new PEOPLExpress, charge for items that will go in the overhead bins. Each airline proposes rules based on the size of their planes and those rules are then certified by the Federal Aviation Administration, an FAA spokeswoman told CNBC. A local FAA cabin safety inspector may periodically appear at the gate to make sure the airlines are enforcing their own rules, she said.
Some of the rules have exceptions—and possibilities for confusion.
For example, Hobica points to a recent announcement from American that states "you can bring a carry-on bag within 45" combined dimensions (including odd-shaped bags)." That appears to conflict with precise dimensions stated elsewhere on its website: "The maximum dimensions cannot exceed any of the following measurements: 22" long x 14" wide x 9" tall or 115cm (56 x 36 x 23 cm)."
One change that American is touting is the addition of allowing garment bags in its cabins. The new policy for American, which recently merged with US Airways, now states: "In lieu of a carry-on bag, you may choose to bring on a soft-sided garment bag of up to 51" in size. You also can bring diaper bags and duty-free items, in addition to a carry-on bag and a personal item, on your flight."
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—By CNBC's Amy Langfield
Follow Road Warrior on Twitter at @CNBCtravel.