I had 2 brooders. I used a rubber maid largest I could find floor space wise, which was pefect for the first week after that it was clear I would need something larger- I planned large enough until transition to coop-
here is a picture- we used scrap wood and left over hardware cloth from constrcution of the coop. We have 5 chicks and it is roughly 2' wide x 3' long and 2' tall. I also wanted the chicks to be able to see out the side, as I had read that makes them less agitated as they can see whats coming at them etc. this set up worked fine with wood pellets, when I added pine shavings, they kicked the shaving all ove the place, so if I ever use it again, I will put some plexiglass up the sides. Also this pic doesnt show the cover, but it is hinges so you can open eighter side, which was important as they tried to fly out every time it was open!!