1sqf of space per chick will keep them until they can go outside. If you want to start with a smaller brooder and keep increasing the size as they get older that is fine too. I start mine in 45 gallon totes. I can put 30 day olds in there and they are good until 2 weeks old. Then they need to be split or put into a larger enclosure. So far I just split them at 2 weeks into 2 brooders. When they start outgrowing that I have an extra large dog crate that they all get put back together in. By the time they outgrow that they are either being sold or put out in the coop with the big girls. I never get my 30 chicks into a 15'x15' enclosure but I make sure there is always plenty of feeder space, space at the water, and little things to keep them busy, like shooter marbles, so they don't pick at each other. Also having roosts so they can go vertical to spread out helps a lot too. Having them in a smaller space does mean you have to clean very frequently to keep them all healthy.
My chickens spend much of their time outside the coop so I only give them 2sqf each of inside floor space, just that much mainly for the days where I can't let them out for whatever reason. My chickens also have two 8' long roosts in their coop which they spend a lot of time on when they are locked in for a day. They say that if you keep your chickens in the coop all the time they need that 4sqf. If you are ranging them in a pen they say 10sqf per chicken. I free range mine most days so they have the ability to wander the whole 2.9 acres I have, although they, by choice, almost never leave the quarter acre barn area. The big chicken factories only have 1sqf of space per hen in their buildings usually.
Now, all that said, that is the space requirements for full size chickens. Depending upon the size of your bantams you could probably get away with smaller spaces easier. I would not go below 2sqf of floor in the coop though, even for bantams unless they are one of the really tiny breeds.
