I was thinking for around 50 or so Dom chicks. If I built it 6 x 3, with the box sides 18 inches and a screened top, that would give me nearly 2600 square inches, of 18 square feet of floor space. That translates to 51 square inches or about 7" x 7" per chick, if all survived and weren't culled. My worry is that it would be too crowded by the time they were fully feathered. I could always build an additional brooder. That being said, I know that if the weather is OK, they could perhaps go out to the pens earlier.....