coop size

I think the recommended size is about a square foot per bird, but to be honest, I would go for more space, as in bad weather, they often like to sit inside, particularly if you incorporate a mesh panel with a sliding front that you can open up during the day.

For 6 hens, I would say 2 nest boxes as well.  Ex-bats never know how to perch and will sleep in nest boxes, which is a messy habit.  You might want to design them so you can shut them off at night