how much room do 2 dozen chicks need?

[mopinko]

antsy to get them, planning to brood them inside, then move them to a closed porch once they are feathered. thinking of getting a recycled baby play pen for them.
they will have a well sheltered coop. at what age would you put them outside? we are having a super mild winter here, but they should be fine outside in march anyway, as long as they are feathered, i think.

i have raised budgies, but these are my first chickens.