Well if you already have a coop you should increase it by a least five times it's size. Bigger is better I always say. It' easier to keep a small flock in a big coop than vice versa.
Anyhow seriously I'd say a good size would be 6' X 6' with a run added or 8' X 8'. Some thing in that square footage. What have you got? Pics would be nice.