As far as RBs go, height is next to meaningless. RB size almost always refers to mass, and generally speaking, the more massive, the more durable. Lots of people thought Reggie Bush would be the next Barry Sanders (all of 5'7.5" and 195 pounds, but he LOOKED much bigger than he was - dude was crazy fit), but while he showed flashes of brilliance, he kept getting dinged up and never came close to realizing his full potential. He was listed at 200 pounds but that was probably in full pads. He was probably closer to 185-190. Mark Ingram isn't nearly as talented as Reggie Bush, but was able to carve himself a solid, if unspectacular career. Ingram is about 5'10" and a bit more massive than Bush, right around 210 pounds or so. Bama's other Heisman tailback, Derrick Henry, is a tight end playing RB. I think in HS he was listed at 6'5" 230 which is absurd for a RB, but his 'true' height of 6'3" was listed at Bama. Normally backs this tall are at a disadvantage because of the higher center of gravity (and often they have less speed), but Henry is an exception because so much of his weight is in his legs. It's really, really hard to bring down a 240-245 pound ball carrier with legs like his. Ron Dayne was as heavy as Henry but much shorter and much slower - you didn't need a fast ball carrier with Wisky's OL. There has to be a good balance of size and speed - I would have to imagine that a 'typical' NFL RB goes roughly 5'11" and 220-225. Still a low-ish center of gravity, and enough mass (especially in the legs) to make him difficult to bring down, but not so much mass that there's a huge speed sacrifice.