Brooder Size

So I'm going to be making my first brooder box this weekend, thing is I'm not sure on the appropriate size.

My plans/thoughts are to keep the chicks indoors for the first three days to make sure all is well before putting them in the garage where the main brooder will be set up. From the garage they will be moved into a pen in the main coop where the other girls can see them, but not yet touch them.

I know that for the most part I've read 2 square feet per chick, my question is this footage used for the entire period of time where they are confided to the brooder? Or are you required to up the square feet as they grow?

We had 24 chicks; our brooder was 5' X 5' X 2.5' It was plenty big for that many chicks, and we could probably have put another 12 in there without too much worry. We made it without a floor, so as they got bigger we just put a block under the edge and they could venture out..just into the shed they were in.... They could also run back in under the heater if they wanted to...week later we opened the outside hatch

My brooder boxes are 4'x4'x2' and I have started 50 chicks in there, by 2 weeks I drop them down to 25/30 chicks, and by 4 weeks they are usually transferred into the brooder shed to give them a lot more room.

I took some pictures of my brooder yesterday with chicks in it. I'll get them on here. Have an awesome day, CynthiaM.