OK,
Recommendations vary from 1/2 square foot to 1 square foot per chick for the first 4 (or 6), weeks.
Then 1-2 square feet per chick for the next 4 (or 6), weeks.
Then about 2 square feet up to about 12 weeks.
Then, as they approach POL, the full 4 square feet like an adult.
Big is good -- as long as the chicks can find their food, their water, and their warm place. I divided my 4x8 brooder in half for the first week or two and found that the moment I let them into the full space they used it all.