Chicks double in size each week, and their space needs double with that growth rate.
A general 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 per chick...
You only need warm the space directly under the lamp. Put your hand under the lamp at floor level. If you can feel warmth, it's warm enough. You are trying to cook your chicks.