Pro Football Prospectus 2007 described RB Vernand Morency as the starter by default, and that he "is a fine receiver out of the backfield and a competent third-down back, but probably isn't suited for full-time duty." At 5'9", 212 lbs., he does seem a little undersized compared to backs like RBs LaDainian Tomlinson (5'10", 221 lbs.), Larry Johnson (6'1", 230 lbs.), or Shawn Alexander (5'11", 225 lbs.).  However he is almost the same size or bigger than RBs like Cedric Benson (5'10", 215 lbs.), Frank Gore (5'9", 217 lbs.), Clinton Portis (5'11", 212 lbs.), and Brian Westbrook (5'8", 203 lbs.). He's no bruiser, but there are several running backs his size doing quite well in the NFL.

Still the Packers are just as likely to get the same production from RBs Noah Herron and Brandon Jackson because no matter which one is starting they are all running behind the same offensive line. From now on Exhibit A in all debates whether a great line makes a great back is RB Edgerrin James who played behind a very good one in Indianapolis in 2005, only to find himself behind one of the worst in Arizona in 2006. From Pro Football Prospectus 2007 page on James:

If the offensive line makes the back, then that is great news for the Packers. With all five starters returning healthy for 2007 and assuming that last season's rookies, Daryn Colledge and Jason Spitz, are improved with a season of NFL experience under their belt, the offensive line should be very good in 2007. I compared them to Philadelphia's young offensive line which took a big step forward in 2006 after all their interior starters had their first experience starting in 2005, anchored by two veterans at tackle. Philadelphia's improved 2006 line might be the single biggest reason RB Brian Westbrook improved from Pro Football Prospectus' #27 best back in 2005 to #6 in 2006.