But rather bigger than smaller because you will invariably be wanting to buy more after you catch the chicken bug!!!! Also, if your coop is too big you can put a large cardboard box on it's side, so then the chooks will hop in there as it is a smaller space to warm. You see, if chickens are...
Generally, make sure there is not so little room in a coop so that they cannot have a little wander, but if the coop is too big, they will not be able to warm the space properly.
I personally would put them in a run with a 6ft fence and wire buried a foot down and a foot out with an overhang at the top but I am in an area with a high concentration of foxes.