Well, Barry Sanders wasn't exactly big...
Amir Pinnix was pretty good for not exactly being a huge RB.
I think the "undersized" argument is a bad one, and I hope that isn't the reason we are not considering him. I heard Drew Brees is "undersized" as well. Guess that explains why Brees is pretty worthless.
Being short has a lot of advantages as a RB, primarily that (1) it's more difficult for the defense to see you behind the linemen, (2) you have a lower center of gravity, making you more difficult to tackle and making cuts easier, physically, and (3) you have less mass to grab ahold of.
If the guy can run, then size is a horrible factor to keep someone away, IMO.