It's interesting to see how the Bruins(and other NHL teams) are kind of all over on this. Bruins have Chara, McQuaid and Carlo yet also run with Krug and Grzelcyk so it seems size IS a factor but it's not the only one.
It's called balance. You need smaller, quick guys, hopefully in your bottom pair that can QB the Power Play, and you need big guys that can use their length and gap control to neutralize speed.
The Bruins will have better balance when McQuaid is back. That should reduce Krug's minutes, which exposes him defensively, and I think Grzelcyk ends up being a healthy scratch most nights, or in Providence.