Welcome to BYC! And yes think of how many birds you want and +3 for extras lol. General rule of thumb is 1ft of roosting space per bird and you want at least a foot of space between the bar and a wall so they can rest comfortably. There are pros and cons to round roosts vs square roosts so recommend that you do some homework on that before you decide.You have pretty good winters insulate well lots of ventilation though
think of how many birds do you want
1 nesting box per 3 hens, add them to the outside so they don't subtract from the inside footprint and also for easier access. Nesting boxes should be below the height of the roosting bars. If the nesting boxes are higher than the roosting bars, the chickens may want to sleep (and leave droppings) in them.
You want plenty of ventilation, but you don't want drafts around their roosting areas. I used expanding insulation foam to fill thin gaps, trimmed with a box cutter, and then painted the outside same as the walls so the chickens wouldn't notice it and peck at it. Use with caution because you DON'T want chickens eating it.
I highly recommend linoleum on the floor, makes cleaning easier and protects the floor. Sand is a flooring option for coop or run, check into it if you want. You can also look into removable poop trays to be placed below the roosts. Also you want the coop door to be small for just the chickens and a bigger door for access and mucking out.
Every hinged opening should have a secure lock. Hardware cloth... secure it with screws and washers, staples can be pulled out or loosened over time. Chicken wire only keeps chickens contained but not predators out.
Good luck, interested to see the finished coop!