A few questions. Where are you brooding them? Are they going to be in a climate controlled area with really steady ambient temperatures or where you can experience temperature swings? To me the best brooders provide a warm enough area in the coldest temperatures and a cool enough temperature in the warmest conditions. That’s pretty easy to do if your brooder is at a constant temperature location, but I brood in my coop. Earlier this year I had a temperature swing from 18 degrees one morning to a high of 80 two days later. I use heat lamps and kept one end warm enough on that 18 degree morning but the other end of my brooder was cool enough at 80 degrees. I also unplugged one of my heat lamps. There are plenty of other ways to provide heat instead of heat lamps but the principle is the same. You need to have one area warm and another cool at all times. That’s really hard to do in a tiny incubator, though the incubator can be much smaller if you are in a steady temperature environment. How big the brooder needs to be depends a lot on where it is and how you provide heat.
How old will the chicks be when you put them outside? They grow extremely fast. What will your weather be like when you are ready to put them outside? Some of us put chicks outside at 5 weeks even in cooler temperatures, others wait much longer. Some ask “How soon can I get these things out of the house”. If you don’t keep the brooder dry, it will stink. The chicks will create a lot of dust anyway, but if the brooder is dry they’ll create a lot more with their scratching. They can be loud, some people like that, some don’t. The brooder needs to be dry for health reasons.
I’ve seen two week old chicks fly very well. You’ll need some type of cover on the brooder to keep them in.
I don’t brood chicks in the house, I never have as few as 6 chicks, and I hardly ever keep them in the brooder past five weeks, six at the most if the outside temperature is going to be much below freezing. From experience I can’t tell you the minimum size you need to make your brooder. I suggest you take some of this into consideration and make it twice as big as you think you need to. That gives you some flexibility in case you guess wrong.
Good luck!