Usain Bolt is pretty tall (6ft 5in), but a lot of other very fast sprinters are not that tall. Tyson Gay and Yohan Blake are both under 6ft, to pick his two likely closest rivals.
Not knowing much about the mechanics of sprinting, I would think a long stride length would be a pretty major advantage in a short sprint. So why aren’t any freakishly tall 7ft+ giants doing great things on the running track? Is it a question of limbs becoming too ungainly to move quickly enough above a certain length, or is it just that height is not a sufficiently great advantage compared with other factors?