I made a 3X7 brooder and it should hold 50 or so for a while and I am building a 4x6 Brooder out of scraps left over in my barn and its about 3/4 the way done. This one should hold 50-65 or so. I'm glad I spread out my orders a few weeks apart! Hopefully by 3 weeks I have them off heat!
Easiest to partition off part of the barn or garage. Yes, they'll need 100 sq ft, I agree. Use the corner of the barn or garage and you'll only need to "build" two sides. Those sides will need to be every bit of 3' high as the 5 week old chicks love to see how high they can jump/fly and perch.