"As much as you can possibly bring yourself to build" is always the smartest plan, IMHO
Seriously. There is NO SUCH THING as having too much ventilation available, although obviously you want to have some mechanism for shutting parts of it down in some weather.
I would suggest building AT LEAST 5 sq ft of vent opening for that size coop, preferably closer to 10 sq ft. I'd say "or more if you're in a hot-summer area" but you're not especially <g>. Make sure that a goodly amount of that vent area is located such taht it will be usable in wintertime, e.g. high on the S wall and protected by roof overhang.
I planned on making most of the ventilation just under the roof structure which will be permanent. I was also planning on making a couple of windows above the nest box which can be closed off in the winter.
It will probably be closer to 20 ft including the windows most of which will be around the eaves to minimise draft.
