I only want to build ONE brooder for 15 chicks

I have 15 chicks varying in age from 5 days to 3 weeks (I only have one 3 week old and she gets along fine with the younger chicks). Most chicks are at the 1.5 to 2 week old range. Currently I have 7 chicks in one rabbit cage and 8 chicks in another rabbit cage. I know they are going to out grow the rabbit cage brooders before long. It is too cold to put them outside and there isn't room in the garage. I have 15 adult birds, so I can't introduce chicks anytime soon. How big of an area should the "grow out" in? How old are they when they typically can withstand nighttime temps in the mid to high 30s?

Thanks!