Sounds like as big as you have room for and can afford is the way to go. I keeping hearing / reading that they do a number on the grass and it's best to rotate them as much as possible, and also helps with disease, etc. I'm sketching a plan as I sit here.
Do you raise any meat birds? Do you know if they need to be kept separate from the layers? It sounds like they may have different feed requirements which would necessitate separating them wouldn't it?
I need some decent weather so I can finish fine tuning the coop - chick time isn't too far off!