For the first week or two chicks need 1/2 square foot each. The space occupied by the feed and water doesn't count.
For the next several weeks they need 1 square foot each.
After that, they need 2 square feet each then up to the full 4 square feet each by the time they're 10-12 weeks.
The bigger the better. After the first few days to a week when they're learning how to find their feed and water they'll make use of more space than you think they will.
Here's how I used my 4'x8'
Outdoor Brooder for my most recent hatch.
First few days, keep them close to the heat and the food.
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After about 5 days, pick up the paper towels and give them a larger area.
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In another week or 10 days -- or when they're starting to think about flying up to the top of the barrier -- give them the entire space.
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They use the entire space as soon as they have access to it.