Airlines Taking Measures to Size Up Your Luggage : Air Travel: Using “sizing boxes” and other gauges, the industry has begun to strictly enforce limits on carry-on bags.

Attention, air travelers who hog overhead luggage space: This could be the fall and winter of your discontent.

Part of this is predictable; the holiday season always brings an increase in carry-on baggage, since many passengers are reluctant to trust fragile gifts to the vagaries of airline and airport luggage handling systems. But this year, that seasonal crunch will be intensified by the airline industry’s new shuttle mentality.

In its efforts to compete with Southwest Airlines and its no-frills service, the Shuttle by United service that opened last month in California has been streamlining boarding procedures and strictly limiting the size and amount of baggage that travelers can carry aboard. USAir did the same about a year ago. Continental Airlines, though its shuttle service is focused in the eastern United States, may tighten enforcement on non-shuttle service as well.

In most cases, the carry-on limits are the same as they have been for years; they’ve just never been strongly enforced. Now, as you arrive at the airport, the first sign of the new carry-on squeeze probably will be a metal-framed “sizing box,” designed to accommodate allowable carry-on items only. The second sign could be a cursing infrequent flyer, who until now has been getting away with flagrant violations of airline limits. A third sign could be the satisfied smiles of travelers who have played by the rules for years, and suffered for it.

“People who bring too much on the aircraft impose on everybody,” said Chris Chiames, spokesman for the Air Transport Assn., a Washington-based group that represents the airline industry..

Flight attendants are glad about the new trend, too. For years they’ve warned about safety threats posed by overloaded bins and told tales of passengers struck on the head, but been given little help by their employers in dealing with the consequences of lax carry-on enforcement. In a 1992 survey of 1,385 flight attendants for various airlines by the Assn. of Flight Attendants, 60% reported that their airline’s carry-on program worked poorly, and 83% called passengers with large bags a major problem. Now, much of the responsibility for enforcing limits has moved to customer service agents at the gates, who supervise the sizing boxes.

There’s still cause for concern among rule-abiding passengers, though, because there is no industry standard on carry-on limits. Most airlines draw the line at two carry-on pieces, but on many commuter flights, the limit is one, and everyone’s dimension limits seem to be just a little different. If you’re facing a flight on an unfamiliar airline and you’re hoping to carry baggage aboard, call to ask the maximum dimensions, and number of carry-on items allowed.

At Southwest Airlines, where sizing boxes have been standard gate equipment since 1971, spokeswoman Linda Burke Rutherford reported no changes in enforcement or customer behavior. She also noted that deep discount fares on Southwest’s California flights have led to full loads, which means a shortage of carry-on room for the last passengers in the boarding line.

“Sometimes, if you’re among the last several people to board, we may be out of overhead bin space,” Rutherford said. If the item can’t be wedged under the seat in front of you, you may be asked to check it. Southwest’s maximum carry-on dimensions: 16 inches in height, 10 inches width, 24 inches length.

In Southern California, the changes are most prominent at United Airlines, which began its Shuttle by United service in Los Angeles on Oct. 1. (Phoenix becomes the 10th city in the system on Dec. 1.) United has placed sizing boxes at all participating airports.

There have been some complaints, acknowledged airline spokesman Tony Molinaro, but United reported no substantial problems. If the first month’s figures from Shuttle by United are an indication, it’s unlikely that the airline will be backing away from these new procedures anytime soon. In its first 30 days, Shuttle by United reported that 71% of its seats were filled, about 5% more than on United’s traditional service. The airline also reported that 96% of its shuttle arrivals were within 15 minutes of the scheduled time--about 8% better than for other United flights.

United’s maximum carry-on dimensions: 14 inches in height, 9 inches in width, 22 inches in length.

At USAir, officials started using sizing boxes nationwide about a year ago, when the airline debuted its own quick-turnaround service. USAir’s maximum carry-on dimensions: 45 inches in combined height, width and length. Unlike its competitors, USAir also specifies a carry-on weight limit: 45 pounds, for up to two pieces combined.

At Continental Airlines, said spokesman David Messing, “The main reason that we want people to be more aware is that we’ve found that stowage problems are contributors to departure delays.” Sizing boxes and other signs of tight enforcement are more obvious at eastern airports served by Continental Lite shuttle flights, but Messing said the airline is hoping to take what it learns about efficiency from Continental Lite and use it in Los Angeles and other destinations. Continental’s maximum carry-on dimensions: 45 inches in combined height, width and length.