I found an interesting article from a few years ago that tried to compare data at the NFL level over the last thirty years to see what impact height and weight have on performance. The data that accompanied the article doesn't show for me, but I thought that the article was interesting, and might stimulate some discussion. Given that the source is BleacherReport, take it with a grain of salt, though.
http://tinyurl.com/4b43gaxOn QBs, they found that additional height had only a slight impact on performance, with the exception that QBs that were exactly 74 inches did better, a statistical anomaly caused by the fact that a number of great QBs were exactly that height (Favre, Young, Montana, Warner). Interestingly, they found a strong relationship between weight and performance, with heavier QB's performing better, and throwing fewer interceptions. The author theorized that it might have to do with willingness to take hits. Overall, though, the best QBs were both taller and heavier.
On RBs they found a relatively weak correlation to height and weight, and that shorter RBs had higher average yardage per carry, and that lighter RBs also performed better than heavier ones, with the exception that heavier RBs were more apt to score TDs. Thus, he found that smaller backs got more yards, but bigger backs got more TDs.
On receivers they found a very strong relationship, with lighter WRs averaging significantly higher yards/catch than heavy WRs. They also found that shorter WRs averaged more yards/catch than tall receivers. Again, though, when it came to TDs, tall receivers did get more TDs. It appears they lumped the tight ends in with the wide receivers, so I suspect it tainted their data significantly, since tight ends average less yds/catch, but more TDs.
Overall, I thought that the article supports that small fast guys are important to have on the team, whether carrying the ball, or catching it.