I hope this is not too late... I said ask the chicken. Really you will likely wish it was bigger as some point. Consider making the run expandable. For example, have one end have an 8 foot wide door on it that easily can be fastened open to make the next wall section. If each wall is made of sections, you could move them with a little effort and perhaps no loss of materials. You could also consider a PVC geodesic dome, easy to move daily. a 100 square foot dome could utilize almost all 500 square feet you have available. Fun fun.To keep it aesthetic I could go as much as 10X50