Just found a great article about fantasy running backs and their prospects, including the fact that they all seam to be shorter stockier players. Most of the top fantasy running backs who had several years of top production were between 5'8" and 5'11" and weighed 205-220lbs.
They had large "cores" (thighs, hips, back/stomach) and low centers of gravity. This allowed them to stand up to the punishment a RB takes and sometimes avoid big hits because of getting a lower pad level than the tackler.
Here is the link to the article-