coop size

It's not possible for the coop to be too large. If you're allowed 20, build for 20. Chicken math afflicts us all.
They don't live in coops because they need to be closely confined. They sleep in coops to keep them safe from predators and out of the rain.
The less densely they're stocked, the easier it is to keep the coop clean and the less chance of disease.
So for 20 chickens, rule of thumb is 80 sq. ft. coop floor space if they have outside access.
Don't forget that 20 chickens need a lot of ventilation and don't freak out too much about the draft thing. Most chickens can live in trees till taken by a predator.

 

The "rule" for coop size is 4 square feet per large fowl chicken. For 20 hens that would be 80 sq ft or 8x10. That's not that huge of a coop. I converted an empty 10x14 stall into my "coop". Started with 8 the first winter and blossomed to 20+ and guineas by the 2nd winter. It's big enough for everyone when the weather is bad but they still get out and wander around the barn during the day so they aren't confined to just the 10x14 area. My run is 10x20 but pretty much never confine them to that. A close call with hawks or coyotes will get them locked up for a few days. The run rule is 10 sq ft per bird if they are locked up. More is always better but less can work if they are free ranging.

I would make your coop as big as you eventually need it. Build it right the first time. If it's nice and big, you will have enough room to set up a dog crate to raise chicks or isolate a hen within the coop easily. Use the lay of the land to tell you how big you can make things. If you are building with reclaimed lumber, use the lumber as your guide for size. Do build your coop at least 6' tall to make your life easier. Put gates into the run so you can get in there as well.

 

Very good advice from PC.
Another consideration is that everyone that keeps chickens should have alternative housing not adjacent to the main coop. For brooding chicks, separating a broody hen or quarantining sick or new birds. It usually doesn't have to be huge but makes life much easier over the long term.

 

I would suggest that you should look at the constraints that you have to build around, then build as big as you would eventually need, so you have to build only once.

If space is limited, the coop can be smaller than the "rule", if there is a full time secure run. A 2x2 coop would not be good for even a flock of 1. Conversely, I kept a flock of 9 in 3x6 coop (full time access to a 6x9 run) with no problem. It was more important that there are adequate roosting space and arrangement in the coop.

Having said the above, coop building might be fun for certain folks. Having two coops does give you flexibility.