The deal is the biggest of its kind for the United States since the North American Free Trade Agreement with Canada and Mexico, which was enacted in 1994. It is Washington’s first bilateral trade pact with a major Asian economy. Studies have estimated that the accord would add $20 billion to bilateral trade, estimated last year at $78 billion. Potential gains to the U.S. economy range from $17 billion to $43 billion, according to Usha Haley, director of the Global Business Center at the University of New Haven. South Korean exports to the United States are expected to rise in the first year by 12 percent, or $5.4 billion.