Actually man, Sciz is right. The prototypical 2-gap 34 d-lineman is 6'5 to 6'7 precicely for the length factor. Why? Because while no one would ever try to ram someone with stiff arms (great visual, btw. Thoughts of Frankenstein), they will frequently lock out a player. Why do you think you see d-linemen in sled drills grip the bag thumbs up and extend? It simulates the punch and grab of the breastplate and the subsequent lockout. You lock a guy out, you stand him up and win the battle. If a lineman reaches your pads it's assumed that you lost. Further, that length enables the ends to see around the tackle or "peek" in vernacular. When a player is playing head-up and two gapping it is critical that they engage, control, identify. If you're gonna be penetrating, you don't need that height, and being in that 6'3 range is advantageous. You said that there are plenty of examples, and this is true, but in completely different 34's. See Pitt, GB, NY, Balt, SD, ect..just because they are 34 defenses doesn't mean that they play the same. Think about the difference between say a Dungy 43 and a Johnson 43. Completely different defenses with completely unique responsibilities.