There is no evidence a back of Bell's size would be a disadvantage, take Steven Jackson for example. He's 6'2 weighs 240 lbs and is having a great pro career. Most backs are in the 5'10 to 6 ft. range but it doesn't mean a taller back can't have success like Steven Jackson demonstrated.
He has all the tools to be a good NFL back thats what you look for when evaluating a prospect, plus the production he had on the field. Yeah, he will face faster, bigger, players in the NFL but you could say the same with any prospect coming out of college.