The magic numbers that everyone mentions are pretty much for a small backyard flock where they are all fully integrated and no more than one boy. It would be a fixed coop and they have reasonable access outside on most days. Even with that set-up, those numbers are overkill for a lot of people, more than they absolutely need. But for some people it's not enough.
Yours is a tractor so you might be able to get by with less room per chicken. It depends on how you manage them. If it stays in one place very long it is not managed like a tractor. Generally you'd need to move it to a new location every day or two. I would not put 15 chickens in that tractor but did you talk to that farmer about how he manages them? If he is moving it twice a day in the summer it might work out.
It will not work in Minnesota in the winter. The ground will be frozen and covered with snow. They will be trapped in that tiny coop section for days on end. That is a recipe for disaster.
With and 8-10 be too big since their body heat keeps them warm?
Your job is not to provide them a warm place to stay. Your job is to provide them a place where they can keep themselves warm. Yes, their body heat keeps them warm, but not by heating up the coop. Their down and feathers trap that body heat. You need to provide them a place that the wind cannot ruffle their feathers so that body heat escapes. If they were free ranging during the day and sleeping in trees or bushes at night they are pretty good at finding places out of the wind. But trapped in our coops and runs they don't have those options, you need to give them an option where they can get out of the wind.
You live in Minnesota, you should know about frostbite. That's the other danger of the cold. If they can stay dry they are not in much real danger of frostbite but if they get damp there is a risk, mainly to combs and wattles. Moisture can come from their breath, their poop, or from their waterer. You need decent ventilation to allow that moisture to escape. The natural inclination is to seal it up tight but that is a mistake. There are different techniques to provide ventilation without allowing a cold wind to hit them but I like having the ventilation openings up high so any winds are over their heads when on the roost or on the floor.
we could enclose the whole thing so that the coop is 8x10 and then make a run for them.
I like the way your husband thinks. In Minnesota in winter you probably need more room than those numbers give. That room could be in the coop or they could include a portion of the run if you can keep it snow free and out of a cold wind. Remember, they don't like a cold wind ruffling their feathers but they can handle cold in a calm area pretty well.
I love that the tractor give them fresh grass but it's definitely not worth it if it's too small. I was leaning this route I just wanted to make sure I wasn't being crazy since that's not what I was told.
It might work depending in how they are managed but it's not working for you now. It certainly will not work this winter. To give them fresh grass next summer you might consider using electric netting to fence in an area that protects them from ground based predators. They are still vulnerable to flying predators but that works for me. This is a good way to use a tractor, move your netting as needed so you don't need to move the tractor as often. If you fence in a large enough area the grass can stay fresh without moving it, they don't wipe it out like they do in a smaller fixed run. If you want to talk detail about electric netting let me know.
If you follow the link in my signature below you can get some of my thoughts on room. Some won't apply to you but some will. I always like to give them as much room as you reasonably can.
Good luck!
