I thought 6'0 was around average statistically speaking, but everything I'm finding right now says average height for an American male is around 5'9.
http://www.cdc.gov/nchs/fastats/bodymeas.htm
Side note, while it shows 5'9 is average for an American male we're on average shorter than a few countries. Germans, Italians, Dutch, etc on average are an inch or more taller. We're also fatter on average.
Back on point though, yes many NFL teams would shy away from a 5'9 corner but then in the NFL players tend to be bigger than college players. In college 5'9 isn't too unusual for corner, it's probably on the low end even for college but not that unusual.