By KORTNEY STRINGER

At the start of a two-week family trip to Paris, Gerry Zinna found herself face-to-face with a Continental Airlines ticket agent armed with a yellow measuring tape. The family's three suitcases were "oversize" -- bigger than Continental allows for checked bags.

Even though the bags were well under the airline's weight limit for luggage, Continental insisted on charging an extra $270 -- per bag. After 30 minutes of yelling across the counter, she reluctantly coughed up an extra $810, money she had earmarked for dining and shopping on her trip. "I was devastated," says the Stockbridge, Mass., resident.

This Labor Day weekend, Americans who take to the skies may be in for a surprise at the ticket counter: A whole generation of luggage, much of which previously might not have raised an eyebrow, is illegal on airlines. While carriers have long restricted carry-on luggage, they have recently begun to enforce long-ignored rules that restrict the size of checked bags as well.

Depending on the size and destination of the luggage, most major carriers charge up to $270 -- often more than the cost of the suitcase itself -- for bags larger than 62 linear inches (defined as height, width and depth added together). The fees can be lower on domestic flights -- usually $80 -- but they can quickly add up since the charges apply per bag per flight segment.

This isn't just leviathan luggage. The suitcases in question often show up in the standard five-bag luggage sets sold by big manufacturers. Indeed, bags as small as 30 inches high (barely kneecap level) can violate the rules if they are stocky enough.

Bag makers say they are continuing to roll out ever-larger suitcase designs because that's what customers have been demanding. Since Sept. 11, such bags have become more popular as passengers try to avoid the hassles of taking a carry-on through tightened security. Skyway Luggage of Seattle says oversize suitcases make up about 10% of its collection. And at Dallas-based Bag 'n Baggage, some of its most popular oversize bags -- between 63 and 64.5 inches -- don't meet airline restrictions. Denver-based Samsonite says one of its latest designs, a wheeled duffel bag, measures in at 64.5 linear inches and is expected to be a big seller.

Stricter enforcement of the checked-luggage policy is one of the most aggressive of the airlines' recent efforts to ramp up revenues by being more diligent about collecting fees for everything from itinerary changes to paper tickets. This month, Delta Air Lines began charging $40 for a third checked bag.

Travelers who fail to check size requirements before the flight have little recourse, because most airlines list the policies on their Web sites. Luggage sellers absolve themselves by warning customers to check with individual airlines for restrictions. And most luggage buyers don't give it a thought. "They generally think it's not so much an issue with checked bags," says Jennifer Carmichael of Skyway Luggage.

To add to the confusion, many retailers don't describe their luggage in terms of linear inches. (Two exceptions: Atlantic Luggage and 1800luggage.com.)

A few bags that are barely legal -- but do make the cut -- include Samsonite's Streamlight 30-inch upright (61.5 linear inches) at samsonitecompanystores.com, and Skyway's Varsity III 29-inch Sky Track expandable suitcase (also 61.5 linear inches) at ebags.com. But watch out: That bag is expandable by three inches, which puts it over the limit.

Technically, all checked luggage, whether it's a hard-sided suitcase or soft-sided garment bag, must meet the size requirements or trigger extra fees. However, soft-sided duffel bags may have a bit more wiggle room since they may be harder to measure.

If traveling light isn't an option, there are a few things passengers can try. While many airlines charge for extra bags, those fees are sometimes lower than the ones on large bags. Take one oversize suitcase on a Continental flight from Newark, N.J., to Paris, for example, and the fee is $180. Take an extra regular size bag instead, and the fee drops to $90. So a traveler may be better off packing an extra suitcase instead of paying for an oversize bag.

The airlines say baggage limitations are a must to accommodate passengers and to ensure the safety of workers who have to lift oversize bags, which tend to weigh more. "We have restrictions like these so that we can accommodate all of our passengers fairly," says Continental Airlines spokeswoman Julie King.

Chris Brathwaite, a spokesman for UAL's United Airlines, said the airline consistently enforces its oversize baggage policy. "If we let someone who shows up with a big bag go [without paying the fees] because they didn't know about our policy, it isn't fair to someone who did check the policy," he said.

That leaves travelers like Cheryl Pulanco feeling bagged. This year, she and her fiance had taken a big black-and-red suitcase on Continental flights for three separate trips to Denver and Oakland, Calif. No charge. Then, on a fourth trip, Continental charged them an extra $160 to cover both legs of the journey.