I've got three 7 week olds in the bottom of a dog crate that's about 24" X 18", but they are out in a pen most of the day and only sleep in there. I'm going to have to put the top back on soon, though, as they are getting too tall for the bottom with a screen across the top! They will have to live with that until they are big enough to integrate with the year old gang.
I think if I didn't have the outside pen I'd want them out - they don't have any room to play in there anymore. It's astounding to think I had 11 of them in there when they were day olds!