What would the dimensions of coop be?

You need about 1 nest box for each 4 hens. For 15 hens, you should be OK with 4 nest boxes.

The standard recommendation on this site is 4 square feet per chicken in the coop along with 10 square feet per chicken in the run. There are a lot of different things that go into this "standard". Some people get by with quite a bit less, but some people get in trouble if they try to squeeze too many chickens in too small a space. Since you are in Maine and plan to restrict them to the coop in the winter, the minimum I'd suggest is 8' x 8'. That gives you 64 square feet total, just over the minimum recommendation. In your circumstances, I'd tend to go bigger, say 8' x 12' since I always believe more is better, but you will probably be OK with 8' x 8'. Brahmas are usually pretty docile and easy-going.

The normal recommendation is 10 square feet per chicken in the run. With 15 chickens, and assuming you make your run 8 feet wide to match your coop, that would be about an 8' x 20' run.

You'll notice I am trying to stay with dimensions divisible by 4 or 8. Most building materials come in those dimensions. If you are careful and pay attention to the difference in center-to-center dimensions versus out-to-out dimensions, you can build something using those dimensions with minimum cutting and minimum waste of material.

I know I am in a much warmer area than you, but my chickens do not mind being outside in single digit fahrenheit weather as long as there is no snow on the ground and the wind is not hitting them. You might consider covering part of your run to help keep the snow out (don't forget how heavy snow is when buiding the cover) and putting up something to block the wind and snow blowing in from the predominant wind directions. They may be able to go outside a lot more than you expect.

 

I agree with ridgerunner on recommendations. And like him (or her), I would at least OFFER your birds an opportunity to go outside daily. If you have more than an inch or two of snow on the ground, they most like won't want to, unless you've shoveled them a clearing. But everyone loves a bit of fresh air, even with it's icy air.

 

Ridgerunner's advice (as usual) is excellent. I heartily agree.