Delta said at the time it announced its plans for the new uniforms that the decision was a "wise investment" that would boost morale among employees and increase the airline's bottom line in the long run.

According to Delta, flight attendants must be able to perform the safety and security functions of their job, including being able to reach and close overhead bins, reach and close any of the aircraft doors and be able to sit on the aircraft jumpseat and fasten the flight attendants seatbelt.

Reller said a flight attendant who wears above a size 18 can do all of those functions well.

"The sizing of our red dress has not been an issue with Delta flight attendants since the program's inception three years ago. This is AFA hype leading up to the election," Laughlin said.

There are roughly 21,000 flight attendants at the combined airline, 14,000 from pre-merger Delta and 7,000 from pre-merger Northwest.

The Northwest workers are represented by a union, while the Delta workers are not. At some point there will be a representation election at which the combined workforce will decide whether it wants to be represented by a union or not. The two groups also must decide on how to integrate their seniority lists.