I didn't even think about size, what I did that was really cheap and easy was take some large cardboard boxes and cut little "doorways" in them, tape the boxes together, and give them a bigger space.
Yeah, I have only had chicks once so far, but I was thinking either general grooming or maybe a bit of boredeom as well. I am not sure how you have your brooder set up, but I gave my chicks little roosts (just made of sticks that I broke into small pieces) and made them a bit higher every so...