That is not neccessarily true. Many times in passing situations, the DE in the 3-4 is responsible for tying up blockers so other can make plays?
Why would I want a former North Carolina Tar Heels basketball player now NFL DE or OLB in the Pats' system dropping into coverage and covering TEs like Antonio Gates who is 6'4" or Dallas Clark who is 6'3"? Other than he can run fast and jump high and is strong enough to fight for balls, I don't know what Peppers could add to the coverage game.
Seriously, guys like Peppers usually become TEs, not DEs. I don't see how his size would hurt his ability to drop into coverage. In fact, I think it would help because it would make it more difficult for the QB to get ball over him even if he is throwing beyond the guy he is covering.
Peppers was a sprinter in high school. He was a high school and college basketball player (and even played in the Final Four). The only reason I could see his height being a problem would be that it would make him slow, but he isn't slow. A fast tall guy is an asset in coverage because it is harder to pass on a tall guy as long as he can cover.