LARGE passengers thinking of flying on that most successful of airlines Southwest might wish to think again. Whilst it has had a "customer of size" policy since 1980, requiring a larger passenger unable to fit in one seat to pay for two it is only in recent times been really implementing it. The airline says it can no longer ignore complaints from slimmer passengers and has begun enforcing the policy more vigilantly requiring passengers to pay for the extra space even if seats were available on the same flight. A refund is made available if it actually takes off with empty seats. With one-fourth of Americans now classified as obese it is a problem. US Airways, Northwest Airlines and America West Airlines all can require an overweight passenger to pay for two seats but say they do everything they can to find a pair of empty adjoining seats on the plane at no additional charge. Each case is an individual judgment call. There are no scales at the check-in counter (as there were many years ago). The test appears to be whether a passenger can sit in one seat without lifting the armrest. In 2000, a California judge ruled that Southwest's policy was "reasonable and not discriminatory" after a woman weighing 300 pounds sued. The woman's civil rights were not violated, the court said. We wait to see what happens this side of the water.