I depends on how long you are going to keep them in there until moving the critters to a bigger place. You'll need about 1 sq. ft. of brooder space for each chick up to 8 weeks. 1/2 size that for bantams. I raised 30 in a 5' x 4' brooder to 8 weeks. Wall to wall chickens by the time they were that age.
I totally agree with azelgin! The smaller the space, the more likely you'll have problems. Chicks could be trampled, or picking could be a problem. Also, smalle chicks could be starved out at feed times. I kept my in 2 smaller groups, in 30 gallon storage totes from the dollar store.