I guess in my limited ( 3 yrs) experience of having chickens, I've learned that a big open run is not ideal. It leaves nothing to discover, and no hiding spots for young birds or the ones lower on the totem pole. I think you could build your breeding pens right into your large space (just divide it up.) If you put a couple of chicken door passageways from pen to pen that can be opened or closed, it becomes a really fun usable space even if you are not in breeding season. Each pen can have its own theme. (food/ grit and water/ exploration/ chilling out). Figure out who you would breed each season, and if you'd do it in one shot or use one pen and just do it in succession. How many birds etc, then you'll know the size. I'm not a great planner, more of a doer- so my pens are always changing. Every year without fail. Love the red coop.