Build it big, and make it so you can easily divide it in case you have a broody or want to introduce new pullets, or have an injured hen, etc.
At one point, I had mine divided into 3 areas, but it was a crazy system of chicken wire and screen that kept falling down. This summer, I want to put up 2x4's to hold a permanent chicken wire divider, with decent human size doors (I was crawling over, under, and had to be a contortionist with the other set up. When it is just the one flock, I'll be able to leave the doors open to maximize space. When I have new pullets or a broody raising chicks, the doors can be closed.
You may not ever plan on having a broody hatch eggs, or introducing new pullets, but I didn't plan on it either. Plans change.
Also, if you live where there is snow, build a secure run with a roof to get them outside in the winter.
Have fun!