He weighs 203 lbs, height doesn't matter. Why do people care about height? Taller running backs actually make bigger targets and take more punishment. 203 lbs is plenty big enough to be the primary ball carrier in the offense, especially when he hasn't even had a full year of a professional workout regimen. Ray Rice entered the league weighing 195 lbs and currently weighs a wopping 9 pounds more than Harris.
Of course, some will ignore the point of that comparison; I'm not saying Harris is as good as Rice, just that Rice has been VERY durable and weighs less than 10 pounds more than Harris. Lesean McCoy is 3 inches taller than Harris but only outweighs him by 8 pounds.
So, could someone please explain to me why Harris is too small to be a primary ball carrier? Other players have done it, ARE doing it. Harris played very well for us last year, did a good job running the ball and was actually a very good receiver during the postseason (something that many forget is that he had 5 receptions for 53 yards against the Vikings). His size didn't seem to hinder him then.