How big does the sleeping coop need to be?

[Shadrach]

I have a multi coop system; related individuals share one coop in general. I have 4 different groups at the moment.
The coops are small and off the ground. The chickens have free choice of which coops they live in.
The most important point is the chickens here free range every day from a about an hour after sunrise to dusk every day no matter what the weather.
I've had a group of hens, 4 in this case, who shared a coop for sleeping and egg laying that gave then less than a foot of floor area each. This is what they chose.
I have another coop where the occupants have a bit more than 3 square feet per fowl (Bantams) and half of the coop doesn't get used.
So, I think that your climate, breed and keeping arrangements are more important than a square footage recommendation.
If you expect weather conditions too harsh to free range in and you have unrelated mixed breeds then 4 square feet is probably not enough space if the chickens are confined for any a number of days or weeks.

 

[Lizzy733]

More is better - go for bigger than needed and you won't regret it. My run is 6mx5m - 30sqm and my coop is sized for 6 birds at 1.2mx2.4m with a little over 2m of perch space. This is currently housing 4, but we will be adding +2 pullets from our recent hatch.
Extra space will give you the flexibility to section off the run if needed for integration\separation\replanting etc... and is better for their wellbeing. They will use all the space you give them so there's no such thing as too much.

 

[J99]

I’m good with plenty of space in my runs but my coop sucks

 

[pennyJo1960]

Then get a small shed at your local hardware framing any slide door to close that to a man door cut another small door either with automatic opener or a pully to open it for them