oh, you did, but on my computer all recent discussions pop up and it doesn't say which section they were posted in until after you click in further, you are posting in the right place!
http://www.backyardchickenelearning.com/what-size-brooder/
It starts out as less than a square foot per chick, but by the time they are 8 weeks they need the 1.5 square feet. I just went with the oldest/biggest amount of space when I first answered as they grow quickly and ultimately that's...
You would need about 1.5 square feet of brooder space per chick up to 8 weeks (generally they are out of the brooder by then), so you would need about 15 square feet of brooder space, so about 3 ft x 5 ft of brooder space