Although smaller RB are becoming more popular, don't knock on bigger guys just because they are an inch or two taller. That same logic means that Westy/MJD can't be good because too small. If the talent and desire are there, then it will happen regardless of size.
The best RBs are squatty because the lower center of gravity affords them superior quickness, fluidity, and overall elusiveness. Consider the recent RBs that are among the NFL's top 30 all-time rushers (sorted by height):Barry Sanders - 5'8" 203 Warrick Dunn - 5'9" 180Tiki Barber - 5'10" 200Thurman Thomas - 5'10" 206Marshall Faulk - 5'10" 211Emmitt Smith - 5'10" 216LaDainian Tomlinson - 5'10" 221Curtis Martin - 5'11" 210Shaun Alexander - 5'11" 225Jerome Bettis - 5'11" 243Ahman Green - 6'0" 218Ricky Watters - 6'1" 211Corey Dillon - 6'1" 225Eddie George - 6'3" 236Average - 5'10.9" 214.6 poundsFunction follows form. I don't think it's any coincidence that hyper durable guys like Emmitt Smith, LaDainian Tomlinson, and Curtis Martin are all almost the exact same size. I believe there is a sweet spot somewhere between 5'10-5'11" and 205-220 pounds that allows for peak RB performance and durability. Only one of the backs on the above list is over 6'1". That back is Eddie George, who really only had five good years before hitting a wall and flaming out completely. Only two of the backs on the above list are over 225 pounds. One of them is George. The other is Jerome Bettis. Bettis had incredible durability for a large back, but I think you have to chalk some of that up to his ideal 5'11" height. Is this conclusive evidence that Adrian Peterson will be a bust? Not at all. Larry Johnson and Steven Jackson are currently two of the most valued RBs in FF. Neither player is in the ideal size range (although, to be fair, neither player has yet proven to be as durable as someone like Emmitt or Martin). Eric Dickerson and Marcus Allen had great careers. They're listed at 6'3" and 6'2" respectively. Each player is a unique event. Trends may apply to a large group, but a given player can always buck the trends. 7th round WRs weren't supposed to have good rookie years. Then Marques Colston beat the odds. I think Adrian Peterson has all of the talent needed to be a great one. But the best of all-time? Highly unlikely, IMO. I suspect he'll have 3-5 good years before hitting the wall ala Eddie George, Jamal Anderson, and Christian Okoye. If you want to talk about the best of all-time, consider #21 on San Diego. He is quite simply the protoypical RB. Pretty much flawless.