I once brooded 28 chicks in a 3’ x 5’ brooder until they were 5 weeks old, then I let them out, just over ½ a square foot per chicken. Most of them were female. Another time I brooded 21 chicks in that same brooder until they were 5 weeks. Most of those were male. In both cases the brooder was getting crowded. I do believe the more room the better but I really don’t believe in magic numbers for much of anything to do with chickens.
I’m not sure where you are located or what your temperatures will be. I don’t know if you plan to brood them in the house, in a separate building like an attached or detached garage, or in the coop. Whatever you do the general principle should be the same. Provide heat at one end, let the rest cool off as it will, and make it big enough so there is a difference. As long as you can keep one area cool enough in the hottest conditions and one area warm enough in the coolest conditions they are really good at self-regulating.
A lot of people do use rubber totes or even aquariums, but the risk is that you overheat them. If you brood in the house you can probably do OK since you don’t have much of a temperature swing from day to night. You can set up your heat so it remains pretty constant. But that won’t work very well if there is much of a temperature change. They are too tight and contained for you to get much difference in one spot to another.
One popular method is to get a cardboard appliance box from your local appliance store. Refrigerator or freezer boxes can work well but you can always tape a couple of boxes together if you need more space. These really work well in a garage. Put something disposable underneath so you don’t get a poop-stained garage floor.
I brood directly in my coop in a 3’ x 6’ brooder. I finally built one in permanently instead of setting up and removing that first one each time. In winter I wrap it with plastic to help hold in the heat. Some mornings I might find ice in the far end but the ends with the heat lamps stay toasty. I’ll include a photo of mine but you may not want to get this permanent.
Looking at those links at the bottom of Azygous’s post can help. She uses a different heating method than I do. There are different things that can work.
I don’t have a recommended size for seven chicks. It depends on what temperatures you are taking about, the temperature swings day to night, and things like that. I don’t know what age you plan to take them out either. That might make a difference. For only seven chicks if you make it big enough to heat one end and let the other cool off you’ll probably be big enough. But I do believe the bigger the better, to a reasonable point.
