Americans will be taller by from one to two inches.

John Elfreth Watkins : engineer
The average American man in 1900 was about 66-67ins (1.68-1.70m) tall and by 2000, the average was 69ins (1.75m). Today, it's 69.5ins (1.76m) for men and 64ins (1.63m) for women.

Tom Geoghegan. "Ten 100-year predictions that came true", BBC News Magazine. 11 Jan 2012.