A friend of a friend with access to the National Health and Nutrition Examinations Survey data looked into the "how many seven footers are there?" question for me and found that the six years of the survey have counted over 3,000 people of whom . . . none are taller than 6 foot 8 inches. Which is to say that "big man" sized people are, in fact, extremely rare. What's more, according to an exhibit I saw at the Mutter Museum last weekend, a majority of people taller than about 6'10" actually suffer from pathological pituitary gland disorders (Sun Ming Ming, for example) that make them ill-suited to be athletes.
The point of this, you'll recall, was to try and estimate what proportion of age-appropriate seven footers are professional basketball players. Perhaps the question should be further refined to include information about this pituitary business. A few super-tall people who, as best I can tell, aren't basketball players are Leonid Stadnyk, Xi Shun, and Ajaz Ahmed. Angus MacAskill at 7'9" was apparently the tallest person recorded without a serious growth disorder, but having been born in Scotland in 1825, basketball wasn't an option.