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I seem to recall that Earl C. was "the first" of he big powerful backs who also had speed.  He was amazing.

I'd choice Barry S. myself because he could gain yards even with poor blocking.

I put Campbell, Herschel, and Bo in that group.  Maybe Ricky Williams?  
Who else belongs with those guys?  Not many, if any.

Earl Campbell was supposedly NFL ready straight out of high school

as was Marcus Dupree
another guy that should easily makes this list, but is a special case because his "career" was short

Back in the day, backs were pretty much big and slow or smaller and shifty/fast.  I should include Jim Brown as a break out player from that mold of course.

Brown would be a solid contender for any top ten list I think.

I favor just admiring the Greats without getting into an unresolvable debate about how they should be ranked.  Is it necessary to have some kind of consensus as to how they should be ranked?  Or perhaps we can just include them as a group of fantastic RBs and let it be.

by weight

think John Riggins

Riggins and Campbell were about 230, but Campbell was 5-11 and Riggins was 6-2. Those are not the same.

Now, for those of you familiar with the Bill James Historical Abstract, he didn't only rank the top 100 at each position, but he wrote a blurb for each one.  Sometimes it was a small paragraph, sometimes several pages, going off on some tangent.  
I'll aim for a blub for each.  

mostly because I'm not a "career" guy - you don't need to prove it to me over 4 seasons and 2,000 carries

I can determine a great back in one season or less

See, I could do this.  It would require more work, and everyone would be placated because it would have Sanders at #1.  We'd have multiple seasons from the same RB, which would be interesting, too.  
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And for those who don't understand my omission of G5 RBs....imagine if an SEC team only had to play 2 SEC games and one OOC game vs a middling P5, like Cal or someone, and the other 9 games were against a bell curve of G5 teams.  You'd all scream from the mountain-top that they shouldn't be held in equal esteem with the other P5 programs...and you'd be correct.
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Let's peek at 1999.  Top rusher:  Dayne.  #2:  Tomlinson (TCU, of the WAC)
Dayne's schedule:
Murray St, Ball St, Cincinnati, 4-Michigan, 12-Ohio St, 25-Minnesota, Indiana, 11-Michigan St, N'Western, 17-Purdue, Iowa, bowl
Tomlinson's schedule:
15-Arizona, N'Western, Arkansas St, Fresno St, San Jose St, Tulsa, Rice, Hawai'i, North Texas, UTEP, SMU, bowl
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These aren't a little different...it's a whole other level.  And it's not even about 5 ranked teams vs 1 or name recognition, forget the top level of the competition.  Look at the middle.  Iowa-Indiana-Cincinnati........hell, who was even the 'middle' level of TCU's schedule?  They're all so random/anonymous, I don't even know.  I don't know if Rice's or Tulsa's or SMU's run defense was decent, because they were probably all shit.  

Back to big backs....some non-bowling ball ones were Eddie George, Eric Dickerson, and Adrian Peterson.  Peterson was the most physical.  I'll never know how George was even good.  Tall and slow.  How can someone be tall AND slow?  But he made it work.  Dickerson had the most upright running style I've ever seen.  Combine that with his huge collar and it looked like he was horse-collaring himself the whole time.
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I prefer the bowling-balls.  P.J. Hill, Anthony Thomas, Ray Rice, etc.
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Bill James also divided hitters into families - based on their singles/doubles/triples/HR ratios.  RB families would be interesting.
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There have been more tall and long RBs than I realized, just perusing the top 100.  
Royster, Peterson, McCaffrey, Etienne, Wheatley, Dickerson, McFadden,.....tall AND big is rare - Wheatley maybe, Derrick Henry has probably been the biggest while being productive.  Kudos to Saban for actually keeping him at RB.  I assume the rest of the world saw a position change for him.