Keep chicks warm for 6 weeks.
After that they will have enough feathers to keep themselves warm.
Also, do not keep them "too warm" after the first two weeks because you want them to grow enough down feathers to keep themselves warm outside.
ALWAYS go as large as you can!
Baby chicks are "cute" for about a week; then they are "cute and messy."
The brooder section of this forum has lots and lots of very good ideas for brooders using items you may already have laying around.
I would venture to say that even a 3x3 isn't going to be big enough for 5-6 chicks. Our *two* are in a child sized playpen. It measures 4 feet wide by 4.5 feet long and 3 feet high. They're 3 weeks old now and flying out of it all the time. They roost on the edge of it, and their poop is enough to make my head spin - has to be cleaned out at least 4x a day. They barely have room in there to move around and there are only 2 of them...plus they are small birds (Norwegian Jaerhons). If you're going to have 5-6 chicks I think the brooder should be at least 6x6 - chicks grow. (but that's my opinion)
I had 10 in a 2.5x4 brooder during the spring and it was FINE for the first 5 weeks, although I did have to clean pretty often. Now, after that, well, lets just say the last two weeks while we waited for the slow girl to get some feathers on her head seemed crowded and they squabbled more than I would have liked. No injuries, they just seemed a bit grumpy in there. Things were much more comfy in the big coop where they could really flap around. I think 2 sq. ft. per chick is plenty, though, unless this will end up as their adult living quarters. Just make sure it has a cover.