From answers.yahoo.com

"AIRCRAFT WEIGHT CLASSES

a. Heavy. Aircraft capable of takeoff weights of more than 255,000 pounds whether or not they are operating at this weight during a particular phase of flight.

b. Large. Aircraft of more than 41,000 pounds, maximum certificated takeoff weight, up to 255,000 pounds.

c. Small. Aircraft of 41,000 pounds or less maximum certificated takeoff weight."

Any aircraft that is classified as a heavy in this manual must use the word heavy when communicating with airtraffic control on or near the airport, with the exception of the Boeing 757, which also must use "heavy".