Thinking about this discussion about Subban, I looked for current NHL defencemen that have overcome size limitations and found:
Kimmo Timonen 5'10", 194 lbs
Tobias Enstrom 5'10", 180 lbs
Neither known for their physicality, but instead hockey smarts, positioning, and offence. Can Subban eventually be that type of player at the NHL level?
Francis Bouillon is another consideration at 5'8", 198. And is actually a physical, stay at home defender. Just a sign that extreme dedication to physical strength can overcome some obstacles, and it looks like Subban is cut from that same cloth, while being smarter, more skilled, and faster.
Rafalski had a fantastic career too at 5'10"
Marc Andre Bergeron (5'9") is inconsistent but can be a powerhouse
Jared Spurgeon (5'9") has had a good start to his career
Brian Campbell, Ian White, Kris Russell, Trevor Daley, JM Liles, Visnovsky have all overcome being 5'10" or less.
Just a few other comperables to look at. But they've all proven that you CAN make it despite being smaller. Sure it's not easy for a smaller player, but if you play in a way that minimizes the handicap and are able to use any combination of speed, strength and intelligence, you can still be very effective.
Barring that, as has been said, he might make a fantastic forward, even just as an energy player with speed and skills to boot.