Every RB that BYU has had that was really good was 5' 11" or more .  There are fast RB in the 6' height range.  I am not saying there aren't good smaller running backs, I am just asking why the change? RB are often used to block, expecially in PRO STYLE offense, in general smaller RB don't make very good blockers in any offense.

Valkenburg  - 6' 2"

Heimuli - 5 11"

Ronny Jenkins - 5  11"

Harvey Unga - 6'

Luke Staley - 6' 1"

Jamal Williams - 6'

Riley Burt - 6' 1"

Peter Tuipulotu - 5  11"

Jamal Willis   6' 3

Jeff Blanc  6'

Curtis Brown 6'