Stage 2 brooder size?

[xtetra]

Rookie chick raiser here. My chicks are about a week and a half old. They are in a large metal tub and are OK for space at the moment. I'm thinking of adding six more chicks before they get any older and am building a larger 2'x4' brooder out of wood for all of them. (Should be finished tomorrow evening.)

So I need to prepare a stage two brooder for when they are bigger in a few weeks right? I was looking for guidelines for space requirements for the stage two brooder but haven't seen any. Maybe I just over-looked them somewhere....

Anyway, any suggestions?