IMHO the best thing for nasty severe cold winters is a LARGE coop, as large as you can stand to build. (I have 10-15 sq ft per chicken indoors, plus a roofed and winter-wind-protected run; from my experiences and for my personal tastes in chicken behavior/happiness, I would not try to go less than that)
Insulate it as much as you can stand, including the ceiling. Design it to capture and retain solar heat well.
Ventilate it well -- yes, you DO need to have ventilation open all winter (unless you're getting down to like -40 F some nights), it actually helps *prevent* frostbite.
Then, if you think you need to add additional heating in the form of a lightbulb or whatever, knock together a (possibly temporary) PARTITION within the coop, like a boxed-in area around the roost or suchlike, so you can heat just THAT part. More economical and less of a fire hazard, for a given air temperature you achieve. And the chickens can decide where to spend their time.
You might take a look at my ventilation and 'cold coop' pages, links in .sig below, for a whole lot more on these topics (I did those pages so I don't have to keep typing it out every time
)
Good luck, have fun,
Pat