My chicks are around 3 weeks old. I've been trying to figure out what to do with them cage wise between the brooder and the outside coop. I haven't seen much on the forums about indoor pen sizes for those weeks. I had some scrap wood around, so I came up with some sort of plan. For the first week I had them in a plastic container right off the kitchen. There is no better way to get to know your new friends than having them flap their wings at the dinner Table. Here is what I came up with for the weeks before they go outside. The rough dimensions are about 2 feet by 4 feet. I put a divider half way between. In another week or so I will remove that and they can have the full length.