'During the summer and early autumn of 2000, Bin Ladin and senior al Qaeda leaders in Afghanistan started selecting the muscle hijackers-the operatives who would storm the cockpits and control the passengers. Despite the phrase widely used to describe them, the so-called muscle hijackers were not at all physically imposing; most were between 5' 5' and 5' 7' in height.'
http://www.9-11commission.gov/report/911Report_Ch7.htm 
