How big is the average running back?
im a fast RB but i think too skinny.
The ideal weight for an RB or a running back is 206-225 pounds. Lighter-weight RBs outperforms the heavier ones in most categories. Some great RBs in history include Barry Sanders, Tiki Barber and Rodney Hampton. Check out more on http://www.sports-central.org/sports/2011/03/15/does_size_matter_for_running_backs.php.
you can't be too skinny to run.
They should be on the lighter side so they can run up and down the field fast.