I never understood this line of thinking. Some of the best defenseman in the NHL are under 6'0. I don't get why people keep putting so much stock into player size.
The game is getting bigger, but you're right, it's really foolish to write off the smaller guys. Karlsson is only 6 feet. As is Doughty, Kronwall, Letang, Del Zotto, Wideman, Subban, Carle. Guys under 6 feet: Boyle, Shattenkirk, Timonen, Campbell, Streit.
All of those guys were in the top 30 defenders for points last seasons. Over a third, so it's a sizable chunk. There's lots of other guys on the list, like Suter or Lidstrom, that are 6'1". So tall enough, but not huge. But the allure of a guy who is huge and can play, like Pronger, Chara, or Weber... it's hard to beat that package. I think that's why size is so valued.
Meanwhile, J-Bo is 6'4" but has the physical game of Jiri Hudler.