I thought I would put together a couple of examples of Dmen who are not considered huge by any means but are huge contributors to their team. 
The team I help coach has dmen who aren’t over-sized so hopefully this helps them visualize. These are two of the most valuable dmen in the greatest league in the world.
Here we go.

Erik Karlsson-6’00” 175lbs.
Kris Letang- 6’00” 195lbs.

Obviously these guys are great players because of how well they skate. But watch these examples and pay attention to the way they: 
-compete
-play hard
-jump up in the play but always recover quickly
-are strong on the puck
-make fast, committed plays
-shoot the puck quickly and have short incredible releases
-get pucks thru to net
-don’t over-handle the puck. Move it quickly and then jump into the play
-are willing to battle bigger players