Chicks double in size each week, and their space needs double each week as well. The good news is that after 6 weeks, the growth rate does begin to slow down.
A basic guideline is as follows -
1 week - 1/4 sq ft per chick
2 weeks - 1/2 sq ft per chick
3 weeks - 1 sq ft per chick
4 weeks - 2 sq ft per chick
5 weeks - 4 sq ft per chick
6 weeks - 8 sq ft
Past 6 weeks, they have about the same space needs as adults. It is much simpler to start with a large enough brooder from the start rather than constantly having to increase the space.