It would depend on the size of the bantam, not all are the same size. Right now I have 15 6 week OEGB in a 3X5X3 brooder pen and they have plenty of room, but they are a small bird and not full grown yet. The pen is open in the daytime and tarped of a night and can be moved daily. Some tractors are not much bigger than the dimensions you gave and is the size of large dog boxes. I also have a rifle crate that is 2X4X2 that is for a hen to sit on eggs. It is small to contain the hens heat, only one hen uses it at a time. As long as my birds have room to forage in the daytime they are happy. I am in the process of building separate pens so each pen will have a separate smaller coop for a few birds.