Well average sized to below average sized defensemen have issues they have to overcome in order to defend effectively at the NHL level. Size is always a concern at that position. You've got to think how is this kid going to be able to handle a Toews, Kopitar, Getzlaf, etc in the corners? How will they handle them in front of the net? How can they defend them? How can they get the puck off them? Can they ever win a puck battle? If they lack size/strength then they'll have to be exceptional with skating and stick work, like world class.
I'd consider 6'0 below average for a defensemen. Any smaller than that and you start to have a lot of concerns on how they will defend. I know people will point to Timonen, Enstrom, or Gostibehere or whoever but the small defensemen in the NHL are the exception not the rule. Not every undersized kid will be able to adequately defend at the NHL level. There's a lot of big, skilled forwards and if they can just stiff arm you or shove you over you're going to get owned repeatedly.
1" might not be a huge difference but there is still a lot of long term concern about how below average sized defensemen will be able to defend in their own zone.
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