I would do a 28 sq. ft. coop for 7 buff Orpington. The run you can choose, but i have heard it is better to has a too big coop than a too small coop.
And also remember, you will need nesting boxes, food, water, grit, and roosts so take in mind where you want that to go.
But, downside to Leghorns, they are not that social. I have 3 Faverolle girls and they are the sweetest things in the world. my favorite girl (a Faverolle) always comes running to me every time i enter the coop. They are so sweet!
Sorry about my posting, im doing to many posts :) XD.