CJ 5'11" 200
Clinton Portis 5'11" 205
Tiki Barber 5'10" 200
Warrick Dunn 5'9" 180
Tony Dorsett 5'11" 192
Thurman Thomas 5'10" 200
And I'm sure plenty more out there. There are simply way too many exceptions to the rule to assume he can't be successful given his size.
I don't think I ever said he can't be successful. However, I think his chances of developing into a 300+ touch workhorse are very low. Dorsett played in a different era when everyone was smaller. He's irrelevant.
Dunn has been a career RBBC back and I'd venture to guess that his actual height is lower than his listed height, giving him a near ideal BMI.
Tiki Barber is listed at 5'10" 205 on NFL.com. Thurman Thomas is listed at 5'10" 206 pounds. That's a big difference from 5'11" 197, even it looks trivial on paper.
Portis is one of the few top backs to gain a significant amount of weight after entering the league. He's up to 220+ now. Also, he was only 20 years old when he entered the league whereas Johnson is already 23 years old. You can't completely rule out the possibility of Johnson gaining weight, but you probably can't assume that it's going to happen. He has a thin frame. Watch him run his 40 yard dash at the combine:
http://www.nfl.com/videos?videoId=09000d5d80737544
Very skinny legs. Compare that to more of a workhorse type back:
http://www.nfl.com/videos?videoId=09000d5d80738aad
Night and day.
Johnson is one of the top athletes at the RB position in this class. A better athlete than Mendenhall. A better athlete than McFadden. The only other top RB prospect in this draft whose overall workout numbers compare favorably with Johnson is Stewart. So he's certainly a freakish athlete. Add that with the first round pedigree and the great training camp hype and there's cause for some excitement.
Nevertheless, I wouldn't advise overlooking his body type shortcomings. His BMI is obscenely low. I'm not sure there's a current top 30 RB in the league who was this small coming out of college. Johnson would have to gain 20 pounds to have a BMI on par with the average top 30 NFL back. I often see him compared to guys like Brian Westbrook and MJD when those comparisons really aren't warranted. Johnson would have to gain 18 pounds to have the same BMI as Westbrook and 38 pounds to have the same BMI as MJD at the same stage of their careers.
There's no way to tiptoe around the issue. He's acutely undersized compared to most successful NFL featured backs. Whether or not that affects your opinion of him is up to you. Personally, it causes me to view him as strictly a change of pace back with very low odds of becoming the type of 300+ touch RB that dominates the hobby.