Ours is built into the coop - it's an area underneath the nesting boxes with a little door. It's about 12 inches high, 15 inches wide, and I think about 2 1/2 feet long- enough for 3 of them to walk around a little but not enough to stretch out and crow. It's meant only for sleeping. We drilled holes in the walls that are inside the coop (for lots of ventilation) but not the outside walls. It stays fairly dark in there as the coop is somewhat dark, so the box is even darker. We put them in around 8p.m. - they let us take them off the roosts and place them in their box. Then I let them out again around 8a.m. They do not crow until I let them out at 8.
However, in the summer, I am afraid they would not get enough ventilation so we will probably bring them in the house (or a cool garage would be good) - people who do that also use a box with drilled holes or a container with a wire lid - the important thing is to keep it dark. Some people say that darkness helps, others don't. Some people say that the low "roof" helps so they can't stretch and crow, others don't. So far, using both in our case is working fine.
Also, I only have 5 hens at the moment but we are adding about 10-12 more soon, as we want to keep all 3 roos and we definitely need more hens to do so.
