That would be the perfect size for them. But, it wouldn't be a bad idea to go bigger. You never know when you will come across another breed you just have to have or one of your hens will go broody and you'll want to let her hatch chicks, etc.
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Try going to your local re-store... run by Habitat for Humanity. They often have all kinds of building materials like doors and windows. We built a coop for our chicken tractor out of hollow core doors.