Welcome to the forum, glad you joined. And thanks for letting us know where you are, your winter weather is important information.
I don't know why you want chickens to start with. Your goals can have an influence on a lot of these answers. I agree that you should look at three as a minimum for the reasons stated. If you ever plan to go with four, start with that. Adding more later, especially with tight space, can be challenging.
Is it humane to keep the chickens in the coop and run 24/7? I can't imagine everyone here has their chickens free roaming their yard -- in fact, our city requires they be enclosed in an area 'primarily for hens'.
Absolutely if you provide them the basics. That includes food, water, protection from predators, and protection form the weather. Adequate room is important, especially with your winters. Another very important thing is keeping the coop and run dry. A wet coop/run can be unhealthy and will almost certainly stink. A dry coop/run should not stink. From what you describe I think keeping them locked up 24/7 is the humane way to go.
Are the outlined coop and run dimensions sufficient? Do I need to go larger? I have a fairly large yard for the city (0.31 acres) but I don't want to take up more space than I need either.
You might follow the link in my signature for some of my thoughts about space. A lot of those will not apply to you but some will. I strongly suggest you plan for the most you plan to have in the future and build for that.
I'm a proponent of providing more space than the absolute minimum. In general I find the more I crowd them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to deal with issues. Why make my life harder than I have to?
Where and how do you plan to feed and water them? Feed needs to stay dry. Can you water outside in your winters? If you feed and water inside you need to be able to locate the feed and water where they will not poop in it from the roosts at night. A 4x4 does not give you much room to do that, especially with only 4' height, but some people manage. It's not just square feet per chicken as some people seem to believe, but do you have the room to arrange things inside. That's tougher with a smaller number of chickens.
In Green Bay you will need more room in winter than someone whose chickens can get outside 365 days a year. That can come in the coop only or if you cover your run in a way to keep snow out and wind blocked the run can count in that. If you can solve the feed/water question and you can use your run in the winter that should be plenty of room for 2 or 4 hens. If you can't depend on the run you might want to go a little bigger on the coop. If you build yourself out of new material most building materials are sold in 4' an 8' dimensions. You can often save a fair amount of cutting and waste if you plan around them. A 6' or similar usually isn't too bad, the cut-off can often be used for nests and such.
Good luck and welcome to the adventure. It can be a fun ride.
