Thanks so much, 3KillerBs! I'm in the Pacific Northwest (Salt Spring Island, BC, Canada) so we get lots of rain in the winter (and weeks of snow that past 2 years), but also drought and heat dome temperatures in the summer.
With that sort of mixed weather, especially if you're subject to high winds during storms, you'll want a flexible design with vents that are well-sheltered. Soffit and ridge and/or gable vents as your main ventilation with additional openings on the leeward side, perhaps with top-hinged covers, are a good option for your situation.
We'd like to start with 6 hens this year and then expand up to 12 in the future. Is there any issue with having a coop that's 'too big' for a little while?
The only time I ever hear people regretting a coop too big is when calculating the cost of the lumber.
The suggested numbers are usually considered minimums. Here's another article I wrote: https://www.backyardchickens.com/articles/over-sized-isnt-just-for-sweaters.77604/
I'd really recommend a walk-in coop for 12 birds.
Likewise.
Once a coop is too big for you to touch every wall from the access door it's best to make it walk-in.