I would think that, for limited use, you could go as small as 10x15 or similar size. One of the rules I've heard for runs is 10 sq feet per bird, so 10x15 would conform to that. I agree with most that, for everyday exclusive use, 10 sq ft per bird would not be enough, but for your purpose I'm assuming A) you want to be economical, and B) you want to make it as safe and predator proof as possible, since you won't be home. A small run is going to be cheaper and easier to get 100% covered- you could even make it narrower and a little longer to make it easier to cover the top.
But I'm not really speaking from experience, here, so feel free to correct me everyone!