I have a shed that is about 10x10. when they get older they could have about 80 sq ft of it. In the mean time, they could have a very cozy garage like area that is about 6x5. Not quite 4 sq ft per bird but maybe when they are small, like up to 6 weeks? I am kind of new to raising chicks so please forgive any obvious wrong headedness on my part. oh yeah, I plan to keep them in a 4'x2'x18" container with screens over the top and a heat lamp for the first couple weeks.
your container will work fine until they feather out then just move them to your 10 x 10 shed. may need to put a heat lamp out there with them depending on your weather.