Thank you for following my thread! My chicken tractor is about 55 square feet or so and okay to house 30 birds. CX don't need as much space because they are bred to be confined in a small space. I personally don't like that idea, so I let mine out every day for several hours so they can explore. They are voracious at hunting for bugs and things.
I believe 8 weeks is the standard butchering age but many do it as early as 6 weeks or let them grow out a little longer. I think it depends on how big you want your dinner to be.
These are the plans we used for our chicken tractor to give you an idea. It cost less than $200 to build and I can move it easily even though I am 5 ft tall. I can say that my birds are way happier out on the grass than they were when I had them in the brooder inside for almost 3 weeks. I'm happier too because I don't have to shovel bedding or smell them!
http://foodcyclist.com/stress-free-chicken-tractor-plans/