Do you think 4x8 with a 4x4 loft is big enough for 12 since they won't get full size and will be out for all waking hours?
Reading the experts, 2 square feet per bird advertised. I tried. For the first few weeks, yes keeping them contained until they were too large to fit through the fence wire. The dogs are very curious.
After 4 weeks, I like seeing them wander around. Hence the garden. Doesn't help the vegetables. But the chickens eat really well. I raised CX. 8 weeks goes fast. They wander. They graze. They GROW and eat!
So, yes, that is large enough. You will want to move it twice a day near the end for fresh grass and to minimize manure load. The CX manure load is a bit higher than the cockerel mix. Still significant. Use your judgement. This is a touchy-feely hobby. Do what feels right.
My answer is always try it. See what your animal husbandry will tolerate. If the gross factor is too high, have a plan to give them more space. Start small and scale up. Mine was the garden. Tractor at night, open to garden during day. I also had a second tractor available. Easier to let them out and sacrifice the remaining veggies. The rain was NOT helping. I kept focus. They are food. I didn't like my food to be gross.
First batch was 6 (supposed to be 10). Second batch was 26. BOY! Glad a started small. Third batch in planning. Leaning towards 20 ish.
The smaller the group, the more you recognize individuals. The larger the group, the more "they" are a flock. But that may just be me.