A coop that’s fine as a bedroom and a hangout/nap time shelter isn’t necessarily going to work for a long winter. I would say 10 sq ft per bird when confined (even if the door is open and they refuse to go out) over long periods such as a week or two on a regular basis. Even then you need hiding places (like a board leaned against a wall... nothing fancy) and ideally pastimes to keep the girls occupied such as (for example) a halved cabbage stuck to a spike (a long nail pounded into the wall from outside) for them to peck at, a rack fastened to a wall that can hold greens, fruits, etc., treats hung by a string from the ceiling... not necessarily all of these things, but pick what appeals to you.
You do need to keep in mind a balanced diet, especially in winter when free range foods are limited, so be reserved in giving them sweets, such as cracked corn or fruits. The general rule I’ve heard is no more than 10% of the diet should be treats.
