How big?

[Alaskan]

With nasty winter, I highly recommend at least 9 square feet per bird of snow free space. Any less than that will result in bad behaviors (cannibalism, egg eating, etc).

It doesn't have to all be coop. .... it can be a coop, and a roofed and sheltered run, and a greenhouse.

My chickens get the greenhouse in the winter. It works out great, and gives them some sun.

 

[McChooky]

I have 6 Dominique pullets in the 30 sq ft predator proof coop and its crowded (bare minimum ) All they have for now is a 48 sq ft covered "grow out pen" attached to the coop until I finish the run. (8 x 16) 120 sq ft .Fortunately my Dominiques get along well but all birds get stressed when kept in small coops and runs. My birds will have total of 200 sq ft once the runs completed but thats not a lot.