A couple of points:
#1 NFL teams, who have access to all of the data in the world, are not concernced that Bush's height/weight ratio will impair his success in the NFL. This is a universally held perspective. The NFL will DISCRIMINATE against stellar college players at various positions to the extent that they are concerned about size -- The 6' QB, the undersized OT, the oversized CB, the Vilma-sized LBs. Thus, the NFL teams front offices WILL discriminate based upon size, yet there is not one team that if they found themselves with pick 1.01 and 5 seconds left on the 15 minute clock, would not shout out - Reggie Bush. End of story on size.
#2 IMO, mega-FF production out of the RB position is largely driven by a confluence of circumstances that are virtually impossible to forecast for RBs entering the NFL draft. Context is more important than Skill. Put a Top 20 RB talent together with a "Best in Class" NFL offense (one that will feature the RB) and you have the best FF RB. Priest Holmes and KC. Marshall Faulk and Stl. Sean Alexander and Sea.
#3 "Busts" at the RB position (and this is a gut feel and someone like Yudkin may run some data that questions the validity of my premise) at the top of the NFL draft are much rarer than at other offensive skill positions such as QB or WR. That is because there is a much higher correlation between "raw measurables & past performance" at the RB position than there is with a QB and WR.
Conclusion: Bush's size is not a problem. His measurables per point #3 justify his draft stake. His ultimate long-term FF prowess will be determined by factors that no one on this board has the capability with which to assess.
To be honest, I would bet that in the end, the risk with top 10 or 15 picks is probably the same at every position. I saw OL mentioned in another thread as the "safest" pick but after looking at the 2000-2005 drafts there was a better chance of getting a good QB than a good OL in the top 10 (11 for Big Ben) picks.As for RBs, here are some guys that I might put on the bust list for RBs taken early since 1985:
2005 Benson #4, Still early, but not looking good so far. Williams and Brown look good, but might not be studs, who knows.
2004 No RBs taken early.
2003 No RBs taken early.
2002 William Green #16, big bust, falls just outside my top 15 cutoff but he was the first RB taken.
2001 No busts. Only LT, stud, taken in top 15.
2000 Thomas Jones #7 (Yes he was decent in 2005, but definitely still a bust based on draft position and the fact that he is his 3rd team). Jamal Lewis was a good pick, not in LT stud territory.
2000 Ron Dayne #11, nuff said.
1999 No busts. EJames and RWilliams were great and good, respectively. RWilliams is more of a bust, just based on personal issues, but was good on the field.
1998 Curtis Enis #5, bust.
1997 No busts. Warrick Dunn is a quality player.
1996 Lawrence Phillips #6, bust.
1996 T. Biakabatuka #8, bust.
1995 Ki-Jana Carter #1, big, big bust since #1 overall. Honorable mention busts to Rashaan Salaam at #21.
1994 No busts. Only stud Marshall Faulk taken in top 15.
1993 Garrison Hearst at #3 overall is an OK pick. He was good overall, but certainly not a stud as some of the top 5 RBs have been. Bettis #10 is a HOFer.
1992 Tommy Vardell #9, bad pick.
1991 Leonard Russell #14, bad pick.
1990 Blair Thomas #2, big, big bust, especially with Emmit Smith going at #17.
1989 Tim Worley #7, too bad Barry Sanders was gone already.
1989 Sammie Smith #9, see above.
1988 Gaston Green #14, first RB taken, still not a good pick.
1987 Damn, all 3 picks in top 15 were bad. Alonzo Highsmith #3, Brent Fullwood #4, D.J. Dozier #14. Awful, awful 1st round for RBs, could be the worst so far, unless you consider Paul Palmer #19, Roger Vick #21, Rod Bernstine #24, and Terrence Flagler #25 quality picks. :X
1986 Only Keith Byars at #10 and John L. Williams #15 were taken early and while they were both good pros, but I would almost consider them busts as RBs. Williams only averaged 600 yards per year over his 8 year career and Byars was a receiving fullback.
1985 No RBs taken early. Honorable mention bust to George Adams the first RB taken at #19.
All in all, there are a good amount of busts with some great players (Edge, LT, Faulk, Sanders) and some good players (R. Williams, J. Lewis, W. Dunn, J. Bettis), but doesn't look like a safe position to me even in the top 15 picks.
Also, RBs don't have as many high profile "busts" because a few years no RB was even picked in the top 15.