Haha I just posted about coop-size remorse. I wish mine was twice as big. And what I would give for a grow pen!
We have three seven-week-old chicks in a cardboard box at night but we're keeping them in a partitioned section of our pen during the day. The cardboard box is just too confining.
I kept forgetting to get a thermometer so I brooded these three by "feel" this time. The first time I did it, I did everything by the book and I honestly think I kept them too warm. All four of them ended up with pasty butt and that was no fun. They all survived but it was several days of worry and unpleasant intervention. I learned a lot by watching my broody hens raise chicks I gave them - they marched them outside for short periods when they were just a few days old and then, of course, warmed them up afterwards. So we've tried to do the same thing with these three. At seven weeks old they are spending the entire day outside - of course we are in California where the temperatures are mild, so that helps!