Yeah, chicken math gets us all. I started with a pre-fab and 6 birds. My flock of 6 turned into 15, a walk in, shed style coop, and a pond is being dug for ducks next year :D . The rule of thumb is 10-12" of roost, per bird. That said, I have 16' of roost and they only use 9' of it. :confused...
Do you have full size chickens in the coop, or are these your first? What are the outside temps where you live? As long as it's warm outside, I usually transfer mine outside around 4-5 weeks. More importantly, you'll want to wean them off the heat source in the brooder. Once they are weaned off...